Land value tax

A land/location value tax (LVT), also called a site valuation tax, split rate tax, or site-value rating, is a levy on the unimproved value of land. It is an Ad valorem tax that, unlike property taxes, disregards the value of buildings, personal property and other improvements. The economic efficiency of a land value tax has been known since the eighteenth century. Many economists since Adam Smith and David Ricardo advocated for this tax, but it is most famously associated with Henry George, who argues that because the supply of land is fixed and the values of land's locations is created by communities and public works, the economic rent of land is the most logical source of public revenue.

A land value tax is a progressive tax, in that the heaviest tax burden would fall on the wealthiest. Land value taxation is currently implemented throughout Denmark, Estonia, Lithuania, Russia, Hong Kong, Singapore, and Taiwan; it has also been applied in subregions of Australia (New South Wales), Mexico (Mexicali), and the United States (Pennsylvania).

Henry George

In his best selling work Progress and Poverty (1879), Henry George argued that because the value of land depends on natural qualities combined with the economic activity of communities, including public investments, the economic rent of land was the best source of tax revenue. A land value tax is said to be a progressive tax, in that the tax burden falls on the owners of valuable land who ostensibly have higher incomes, and because LVT cannot be passed on as higher costs or lower wages to tenants, consumers or workers. The philosophy that natural resource rents should be captured by society is now often known as Georgism. Its relevance to public finance is underpinned by the Henry George theorem.

Economic effects

The value of land (and many other macro-economic quantities) can be expressed in two ways. Land value is directly related to the value it can provide over a certain period of time, also known as ground rent. The capitalization of this ground-rent by the land market is what creates land prices, the other measure of land value. When ground-rent is redirected to the public, through LVT for example, the price of land will decrease, holding all else constant. The rent charged for land also decreases as a result of efficiency gains from the ad valorem aspect of LVT.

Land that is not desirable enough for use at a given time is called marginal land. Land value tax causes the quality of marginal land would improve while also decreasing in rent, as explained by Ricardo in 1816.

Efficiency

File:Perfectly inelastic supply.svg

A supply and demand diagram showing the effects of land value taxation. If the supply of land is fixed, the burden of the tax falls entirely on the land owner, with zero or even negative deadweight loss.

Most taxes distort economic decisions and suppress beneficial economic activity. LVT is payable regardless of how well or poorly land is actually used. Because the supply of land is essentially fixed, land rents depend on what tenants are prepared to pay, rather than on landlord expenses, preventing landlords from passing LVT to tenants.

The direct beneficiaries of incremental improvements to the area surrounding a site are the land's occupants. Such improvements shift tenants' demand curve to the right. Landlords benefit from price competition among tenants; the only direct effect of LVT in this case is to reduce the amount of socially generated benefit that is privately captured (as an increase in the land price).

LVT is said to be justified for economic reasons because it does not deter production, distort markets or otherwise create deadweight loss. Land value tax can even have negative deadweight loss (social benefits), particularly when land use improves. Nobel Prize-winner William Vickrey believed that "removing almost all business taxes, including property taxes on improvements, excepting only taxes reflecting the marginal social cost of public services rendered to specific activities, and replacing them with taxes on site values, would substantially improve the economic efficiency of the jurisdiction." A positive relationship of LVT and market efficiency is predicted by economic theory and has been observed in practice.

Fred Foldvary stated that the tax encourages landowners to develop vacant/underused land or to sell it. He claimed that because LVT deters speculative land holding, dilapidated inner city areas return to productive use, reducing the pressure to build on undeveloped sites and so reducing urban sprawl. For example, Harrisburg, Pennsylvania in the United States has taxed land at a rate six times that on improvements since 1975. This policy was credited by mayor Stephen R. Reed with reducing the number of vacant structures in downtown Harrisburg from around 4,200 in 1982 to fewer than 500. LVT is arguably an ecotax because it discourages the waste of prime locations, which are a finite natural resource.

LVT is an efficient tax to collect because unlike labour and capital, land cannot move to escape tax.

Many urban planners claim that LVT is an effective method to promote transit-oriented development.

Real estate values

Real estate bubbles direct savings towards rent seeking activities rather than other investments and can contribute to recessions. Advocates claim that it reduces the speculative element in land pricing, thereby leaving more money for productive capital investment.

At sufficiently high levels, land value tax would cause the real estate prices to fall by removing land rents that otherwise would become 'capitalized" into the price of real estate. It also encourages landowners to sell or relinquish titles to locations that they are not using. This provides them strong incentives for landowners to prevent such confiscatory policies. Since landowners often possess political influence, they have generally been able to prevent such outcomes.

Practical issues

Several practical issues are involved in the implementation of a land value tax. Most notably, it must be:

• Calculated fairly and accurately
• High enough to raise sufficient revenue without causing land abandonment
• Billed to the correct person or business entity

Assessment/appraisal

Compare to Real estate appraisal

See also: Land (economics)

Levying a land value tax is straightforward, requiring only a valuation of the land and a title register. Value assessment can be difficult in practice. In a 1796 United States Supreme Court opinion, Justice William Paterson noted that leaving the valuation process up to assessors would cause bureaucratic complexities, as well as non-uniform assessments. Murray Rothbard later raised similar concerns, stating that no government can fairly assess value, which can only be determined by a free market.

However, as Steven Spadijer pointed out, markets already provide value assessments. Land sold independently of improvements provides direct evidence. Value can be computed by capitalizing rental streams. Further, valuators in the home-and-contents insurance industry do so in order to calculate an insurance premium, separating the value of the home from the value of the land. Another approach is the residual method: the value of the site is the property's total value minus the depreciated value of buildings and other structures.

Compared to modern day property tax evaluations, land valuations involve fewer variables and have smoother gradients than valuations that include improvements. This is due to variation of building style, quality and size between lots. Modern statistical techniques have eased the process; in the 1960s and 1970s, multivariate analysis was introduced as an assessment means. Usually, such a valuation process commences with a measurement of the most and least valuable land within the taxation area. A few sites of intermediate value are then identified and used as "landmark" values. Other values are filled in between the landmark values. The data is then collated in a database and linked to a unique property reference number, "smoothed" and mapped using a geographic information system (GIS). Thus, even if the initial valuation is difficult, once the system is in use, successive valuations become easier.

Revenue

File:Maximum taxation with perfectly inelastic supply.svg

In this case, land is taxed at 100% of its value, eliminating the landowner surplus completely.

In the context of land value taxation as a single tax (replacing all other taxes), some have argued that LVT alone cannot raise enough revenues. However, the presence of other taxes can reduce land values and hence the amount of revenue that can be raised from them. The Physiocrats argued that all taxes are ultimately at the expense of land rental values. Most modern LVT systems are alongside other taxes and thus only reduce their impact without removing them. Land taxes that are higher than the rental surplus (the full land rent for that time period) would result in landowner abandonment.

Title

In some countries, LVT is impractical because of uncertainty regarding land titles and established land ownership and tenure. For instance a parcel of grazing land may be communally owned by village inhabitants and administered by village elders. The land in question would need to be held in a trust or similar body for taxation purposes. If the government cannot accurately define ownership boundaries and ascertain the proper owners, it cannot know from whom to collect the tax. The lack of clear titles is found in many developing countries. In African countries with imperfect land registration, boundaries may be poorly surveyed and the owner can be unknown. LVT proponents argue that such owners can be made to identify themselves under penalty of losing the land.

Incentives

Speculation

The owner of a vacant lot in a thriving city must still pay a tax and would rationally perceive the property as a financial liability, encouraging him/her to put the land to use in order to cover the tax. LVT removes financial incentives to hold unused land solely for price appreciation, making more land available for productive uses. Land value tax creates an incentive to convert these sites to more intensive private uses or into public purposes.

Incidence

The selling price of a good that is in fixed supply, such as land, decreases if it is taxed. By contrast, the price of manufactured goods can rise in response to increased taxes, because the higher price reduces the number of units that are made. The price increase is how the maker passes along some part of the tax to consumers. However, if the revenue from LVT is used to reduce other taxes or to provide valuable public investment, it can could cause land prices to rise as a result of higher productivity, by more than the amount that LVT removed.

Land Tax incidence rests completely upon landlords, although business sectors that provide services to landlords are indirectly impacted. In some economies, 80 percent of bank lending finances real estate, with a large portion of that for land. Reduced demand for land speculation might reduce the amount of circulating bank credit.

While owners cannot charge higher rent to compensate for LVT, removing other taxes may increase rents.

Land use

Assuming constant demand, an increase in constructed space decreases the cost of improvements to land such as houses. Shifting property taxes from improvements to land encourages development. Such infill of underutilized urban space also combats sprawl.

Collection

LVT is less vulnerable to tax evasion, since land cannot be moved overseas and titles are easily identified, as they are registered with the public. Land value assessments are usually considered public information, which is available upon request. Transparency reduces tax evasion.

Ethics

See also: Georgism

Land acquires a scarcity value owing to the competing needs for space. The value of land generally owes nothing to the landowner and everything to the surroundings. LVT supporters claim that the value of land depends on the community.

Religion

In religious terms, it has been claimed that land is God's gift to mankind. For example, the Roman Catholic Church as part of its Universal destination of goods principle asserts:

Everyone knows that the Fathers of the Church laid down the duty of the rich toward the poor in no uncertain terms. As St. Ambrose put it: "You are not making a gift of what is yours to the poor man, but you are giving him back what is his. You have been appropriating things that are meant to be for the common use of everyone. The earth belongs to everyone, not to the rich."

— Pope Paul VI, Populorum Progressio (1967)

In addition, the Church maintains that political authority has the right and duty to regulate, including the right to tax, the legitimate exercise of the right to ownership for the sake of the common good.

Equity

LVT considers the effect on land value of location, and of improvements made to neighbouring land, such as proximity to roads and public works. LVT is the purest implementation of the public finance principle known as value capture.

A public works project can increase land values and thus increase LVT revenues. Arguably, public improvements should be paid for by the landowners who benefit from them. Thus, LVT captures the value of socially created wealth, allowing a reduction in tax on privately created (non-land) wealth.

LVT generally is a progressive tax, with those of greater means paying more, in that land ownership is correlated to incomes and landlords cannot shift the tax burden onto tenants. LVT generally reduces economic inequality removes incentives to misuse real estate and reduces the vulnerability of economies to property bubbles and their collapse.

History

Pre-modern

Land value taxation began after the introduction of agriculture. It was originally based on crop yield. This early version of the tax required simply sharing the yield at the time of the harvest, on a yearly basis.

Aryan sages of ancient India claimed that land should be held in common and that unfarmed land should produce the same tax as productive land. "The earth ...is common to all beings enjoying the fruit of their own labour; it belongs...to all alike"; therefore, "there should be left some for everyone". Apastamba said "If any person holding land does not exert himself and hence bears no produce, he shall, if rich, be made to pay what ought to have been produced".

During the Middle-Ages, in the West, the first regular and permanent land tax system was based on a unit of land known as the hide. The hide was originally an amount of land sufficient to support a household, but later became subject to a land tax known as "geld".

Mencius was a Chinese philosopher (around 300 BCE) who advocated for the elimination of taxes and tariffs, to be replaced by the public collection of urban land rent: "In the market-places, charge land-rent, but don't tax the goods."

Physiocrats

Anne Robert Jacques Turgot, one of the leading physiocrats.

The physiocrats were a group of economists who believed that the wealth of nations was derived solely from the value of land agriculture or land development. Physiocracy is one of the "early modern" schools of economics. Physiocrats called for the abolition of all existing taxes, completely free trade and a single tax on land. They did not distinguish between the intrinsic value of land and ground rent. Their theories originated in France and were most popular during the second half of the 18th century. The movement was particularly dominated by Anne Robert Jacques Turgot (1727–1781) and François Quesnay (1694–1774). It influenced contemporary statesmen, such as Charles Alexandre de Calonne. The Physiocrats were highly influential in the early history of land value taxation in the United States.

Radical movement

A participant in the Radical movement, Thomas Paine contended in his Agrarian Justice pamphlet that all citizens should be paid 15 pounds at age 21 "as a compensation in part for the loss of his or her natural inheritance by the introduction of the system of landed property." "Men did not make the earth. It is the value of the improvements only, and not the earth itself, that is individual property. Every proprietor owes to the community a ground rent for the land which he holds." This proposal was the origin of the citizen's dividend advocated by Geolibertarianism. Thomas Spence advocated a similar proposal except that the land rent would be distributed equally each year regardless of age.

Classical economists

Adam Smith, in his 1776 book The Wealth of Nations, first rigorously analyzed the effects of a land value tax, pointing out how it would not hurt economic activity, and how it would not raise land rents.

Ground-rents are a still more proper subject of taxation than the rent of houses. A tax upon ground-rents would not raise the rents of houses. It would fall altogether upon the owner of the ground-rent, who acts always as a monopolist, and exacts the greatest rent which can be got for the use of his ground. More or less can be got for it according as the competitors happen to be richer or poorer, or can afford to gratify their fancy for a particular spot of ground at a greater or smaller expense. In every country the greatest number of rich competitors is in the capital, and it is there accordingly that the highest ground-rents are always to be found. As the wealth of those competitors would in no respect be increased by a tax upon ground-rents, they would not probably be disposed to pay more for the use of the ground. Whether the tax was to be advanced by the inhabitant, or by the owner of the ground, would be of little importance. The more the inhabitant was obliged to pay for the tax, the less he would incline to pay for the ground; so that the final payment of the tax would fall altogether upon the owner of the ground-rent.

— Adam Smith, The Wealth of Nations, Book V, Chapter 2, Article I: Taxes upon the Rent of Houses

Georgism

Henry George in 1865.

Main article: Georgism

Henry George (2 September 1839 – 29 October 1897) was perhaps the most famous advocate of recovering land rents for public purposes. An American journalist, politician and political economist, he advocated a "Single Tax" on land that would eliminate the need for all other taxes. In 1879 he authored Progress and Poverty, which significantly influenced land taxation in the United States and other countries, including Denmark, which continues 'grundskyld' (Ground Duty) as a key component of its tax system.

Meiji Restoration

After the 1868 Meiji Restoration in Japan, Land Tax Reform was undertaken. A land value tax was implemented beginning in 1873. By 1880 initial problems with valuation and rural opposition had been overcome and rapid industrialisation began.

Liberal and Labour Parties in the United Kingdom

In the United Kingdom, LVT was an important part of the platform of the Liberal Party during the early part of the twentieth century: David Lloyd George and H. H. Asquith proposed "to free the land that from this very hour is shackled with the chains of feudalism." It was also advocated by Winston Churchill early in his career. The modern Liberal Party (not to be confused with the Liberal Democrats, which are the heir to the earlier Liberal Party and who offer some support for the idea) remains committed to a local form of land value taxation, as do the Green Party of England and Wales and the Scottish Green Party.

The 1931 Labour budget included a land value tax, but before it came into force it was repealed by the Conservative-dominated National Government that followed shortly after.

An attempt at introducing site value taxation in the administrative County of London was made by the local authority under the leadership of Herbert Morrison in the 1938–9 Parliament, called the London Rating (Site Values) Bill. Although it failed, it detailed legislation for the implementation of a system of land value taxation using annual value assessment.

After 1945, the Labour Party adopted the policy, against substantial opposition, of collecting "development value": the increase in land price arising from planning consent. This was one of the provisions of the Town and Country Planning Act 1947, but it was repealed when the Labour government lost power in 1951.

Senior Labour figures in recent times have advocated an LVT, notably Andy Burnham in his 2010 leadership campaign, current Leader of the Opposition Jeremy Corbyn and Shadow Chancellor John McDonnell.

Modern economists

Alfred Marshall argued in favour of a "fresh air rate", a tax to be charged to urban landowners and ‘‘levied on that value of urban land that is caused by the concentration of population’’. That ‘‘general rate’’ should have ‘‘to be spent on breaking out small green spots in the midst of dense industrial districts, and on the preservation of large green areas between different towns and between different suburbs which are tending to coalesce’’. This idea influenced Marshall's pupil Arthur Pigou's ideas on taxing negative externalities.

Paul Samuelson supported a land value tax. "Our ideal society finds it essential to put a rent on land as a way of maximizing the total consumption available to the society. ...Pure land rent is in the nature of a 'surplus' which can be taxed heavily without distorting production incentives or efficiency. A land value tax can be called 'the useful tax on measured land surplus'."

Milton Friedman stated: "There's a sense in which all taxes are antagonistic to free enterprise – and yet we need taxes. ...So the question is, which are the least bad taxes? In my opinion the least bad tax is the property tax on the unimproved value of land, the Henry George argument of many, many years ago."

Michael Hudson was a proponent for taxing rent, especially land rent.".... politically, taxing economic rent has become the bête noir of neoliberal globalism. It is what property owners and rentiers fear most of all, as land, subsoil resources and natural monopolies far exceed industrial capital in magnitude. What appears in the statistics at first glance as "profit" turns out upon examination to be Ricardian or "economic" rent."

Paul Krugman agreed that a land value tax is efficient, however he disputed whether it should be considered a single tax, as he believed it would not be enough alone, excluding taxes on natural resource rents and other Georgist taxes, to fund a welfare state. "Believe it or not, urban economics models actually do suggest that Georgist taxation would be the right approach at least to finance city growth. But I would just say: I don’t think you can raise nearly enough money to run a modern welfare state by taxing land [only]."

Nobelist Joseph Stiglitz wrote, "Not only was Henry George correct that a tax on land is non-distortionary, but in an equilibrium society ... tax on land raises just enough revenue to finance the (optimally chosen) level of government expenditure."

Canada

Land value taxes were common in Western Canada at the turn of the twentieth century. In Vancouver LVT became the sole form of municipal taxation in 1910 under the leadership of mayor, Louis D. Taylor. Gary B. Nixon (2000) stated that the rate never exceeded 2% of land value, too low to prevent the speculation that led directly to the 1913 real estate crash. All Canadian provinces later taxed improvements.

Twenty-first century

Existing tax systems

Land value tax systems are in use in multiple countries.

Australia

The Henry Tax Review recommended that state governments replace stamp duty on the sale of land with LVT, without exemption for a person's principal place of residence or farmland. The review proposed multiple marginal rates and that most agricultural land would be in the lowest band with a rate of zero. Only the Australian Capital Territory moved to adopt this system and planned to reduce stamp duty by five percent and raise land tax by five percent for each of twenty years.

The state of New South Wales levies a state land value tax. However, unlike council rates, farmland and principal residences are generally exempt. The state tax is levied only on value over a certain threshold. Determination of land value for tax purposes is the responsibility of the Valuer-General.

Sydney, Canberra and other jurisdictions in Australia use LVT. A government report in 1986 for Brisbane, Queensland advocated an LVT.

By revenue, property taxes represent 4.5% of total taxation in Australia.

United States

Main article: Land value tax in the United States

Common property taxes include land value, which usually has a separate assessment. Thus, land value taxation already exists in many jurisdictions. Some jurisdictions have attempted to rely more heavily on it. In Pennsylvania certain cities raised the tax on land value while reducing the tax on improvement/building/structure values. For example, the city of Altoona adopted a property tax that solely taxes land value in 2002.

In the late 19th century George followers founded a single tax colony at Fairhope, Alabama. Although the colony, now a nonprofit corporation, still holds land in the area and collects a relatively small ground rent, the land is subject to state and local property taxes.

Hong Kong

Government rent in Hong Kong, formerly the crown rent, is levied in addition to Rates. For properties that are located in the New Territories (including New Kowloon), or located in the rest of the territory and whose land grant was recorded after 27 May 1985, government rent is levied at 3% of the rateable rental value.

Other countries

Pure LVT, apart from real estate or generic property taxation, is used in Taiwan, Singapore and Estonia. It has also been used in Mexicali, Mexico.

Countries with active discussion

China

China's Real Rights Law contains provisions founded on LVT analysis.

Ireland

In 2010 the government of Ireland announced that it would introduce an LVT, beginning in 2013. However following a 2011 change in government, a property value tax was introduced instead (see Local property tax (Ireland)).

Kenya

Kenya's LVT history dates to at least 1972, shortly after it achieved independence. Local governments must tax land value, but are required to seek approval from the central government for rates that exceed 4 percent. Buildings were not taxed in Kenya as of 2000. The central government is legally required to pay municipalities for the value of land it occupies. Kelly claimed that maybe as a result of this land reform, Kenya became the only stable country in its region. As of late 2014, the city of Nairobi was still taxing only land values, although a tax on improvements had been proposed.

Namibia

A land value taxation on rural land was introduced in Namibia, with the primary intention of improving land use.

United Kingdom

Labour Land Campaign activities within the Labour party and the broader Labour movement for "a more equitable distribution of the Land Values that are created by the whole community" through LVT. Its membership includes members of the British Labour Party, Trade Unions and Cooperatives and individuals. The Liberal Democrats' ALTER (Action for Land Taxation and Economic Reform) aims

to improve the understanding of and support for Land Value Taxation amongst members of the Liberal Democrats; to encourage all Liberal Democrats to promote and campaign for this policy as part of a more sustainable and just resource based economic system in which no one is enslaved by poverty; and to cooperate with other bodies, both inside and outside the Liberal Democrat Party, who share these objectives.

The Green Party "favour moving to a system of Land Value Tax, where the level of taxation depends on the rental value of the land concerned."

Courses in "Economics with Justice" with a strong foundation in LVT are offered at the School of Economic Science, which has historical links with the Henry George Foundation.

Scotland

Since the beginning of the twenty-first century, with devolution and the re-establishment of the Scottish Parliament, interest and political pressure in Scotland has grown.

In February 1998 the pre-devolution UK government in Scotland (the Scottish Office) launched a public consultation process on land reform. A survey of the public response found that: "excluding the responses of the lairds and their agents, reckoned as likely prejudiced against the measure, 20% of all responses favoured the land tax" (12% in grand total, without the exclusions). The government responded by announcing "a comprehensive economic evaluation of the possible impact of moving to a land value taxation basis". However, no measure was adopted.

In 2000 the Parliament’s Local Government Committee's inquiry into local government finance explicitly included LVT, but the final report omitted any mention.

In 2003 the Scottish Parliament passed a resolution: "That the Parliament notes recent studies by the Scottish Executive and is interested in building on them by considering and investigating the contribution that land value taxation could make to the cultural, economic, environmental and democratic renaissance of Scotland."

In 2004 a letter of support was sent from Scottish Parliament MPs to the organisers and delegates of the IU’s 24th international conference—including members of the Scottish Green, Socialist and Nationalist parties.

The policy was considered the 2006 Scottish Local Government Finance Review whose 2007 Report concluded that "although land value taxation meets a number of our criteria, we question whether the public would accept the upheaval involved in radical reform of this nature, unless they could clearly understand the nature of the change and the benefits involved.... We considered at length the many positive features of a land value tax which are consistent with our recommended local property tax [LPT], particularly its progressive nature." However, "[h]aving considered both rateable value and land value as the basis for taxation, we concur with Layfield (UK Committee of Inquiry, 1976) who recommended that any local property tax should be based on capital values."

In 2009, Glasgow City Council resolved to introduce LVT: "the idea could become the blueprint for Scotland’s future local taxation" The Council agreed a "long term move to a local property tax / land value tax hybrid tax": its Local Taxation Working Group stated that simple [non-hybrid] land value taxation should itself "not be discounted as an option for local taxation reform: it potentially holds many benefits and addresses many existing concerns".

Policy interest

In Zimbabwe, government coalition partners the Movement for Democratic Change adopted LVT. Since 2000, political expressions of interest in the policy emerged in Belgium, Ethiopia, Republic of South Africa and other countries. The governments of Thailand and Hungary expressed some interest in the policy.

Local campaigns are active in other countries, including South Korea. The IU works internationally and at the United Nations in support of the policy. In 1990, several economists wrote to then President Mikhail Gorbachev suggesting that Russia adopt LVT; its failure to do so was argued as causal in the rise of the Oligarchs.

Tax rates

EU countries

Country	Average rate	Lowest rate	Highest rate	Year	Name	Description
Denmark	2.627%	1.6%	3.4%	2013	grundskyldspromille / ejendomsskat	The municipality (kommune) decides the local tax rate within 1.6 and 3.4 percent

See also

• Citizen's dividend
• Classical economics
• Ecotax
• Equity in taxation
• Geographic information system
• Geolibertarianism
• Georgism
• International Union for Land Value Taxation (The IU)
• Land (economics)
• Land monopolization and reform
• Land speculation
• Land tenure and registration
• Law of rent
• Lockean proviso
• Natural resource economics
• Optimal tax
• Physiocracy
• Pigovian tax
• Progressive tax
• Property rights (economics)
• Rent-seeking
• Single tax
• Tax reform
• Tax shift
• Value capture
• Wealth tax

References

Sources

• Coughlan, J. Anthony. "Land Value Taxation and Constitutional Uniformity", Geo. Mason L. Rev., Winter 1999, Vol. 7, No. 2

External sources

• Land Tax Management Act 1956: New South Walesfr:Impôt unique