"We shall can easily overcome"

From Chapter 20 "Conclusions and Recommendations for Reform" of the Mirrlees Review:

Pure economic rents can, in principle, be taxed without creating an economic distortion. One example is the ‘excess’ return to capital; taxing such returns does not discourage saving and investment. In practice, it can be difficult to pinpoint rents, and so we are wary of attempting to tax them at higher rates than ordinary income.

But where rents can be identified accurately, targeted taxes can be applied. In particular, there is a strong case for levying a land value tax, which is a tax on pure rent—if the practical difficulty of valuing land separately from the buildings on it can be overcome.

Valuation is the easiest thing in the world, HM Land Registry has got 99% of the information it needs all held in a database to give relative values for every single patch of land to within +/- ten per cent at the touch of a button. There is no perfect 100% mathematically and scientifically correct answer, but the same applies to any tax (most existing taxes are dictated by politics/special interest groups rather than economic logic anyway).

So to some extent, LVT rates are trial and error (as are any tax rates), but it's easy to spot if the rate (in £-s-d) is too high in any area; land and buildings in that area - however new or well maintained - would sell for less than their rebuild cost (which is routinely established by insurance companies).

As long as land and buildings in any area are selling for noticeably more than their rebuild cost - and as long as owners of undeveloped and derelict sites choose to keep rather than abandon them - the LVT imposed in that area (in absolute £-s-d figures) is clearly less than 100% of the economic rent, so no harm done.
--------------------------------------
Footnote to illustrate the point, for the benefit of The Fat Bigot and others:

We all know full well that UK housing can be sub-classified into a few generic types, the most important one being three-bed semi-detached houses We establish that In the UK semi-detached houses are the most common property type, accounting for 32% of UK housing transactions and 32% of the English housing stock as of 2008.

We all know what they look like, a bit like this:There are small variations (bay window, round, diagonal, none), built on garage (yes, no), nice bricks, pebble-dashed or pre-fab out of concrete, the smallest is maybe a fifth smaller than the biggest etc. but the basic layout is always the same, they were nearly all built between 1930 and 1970 and the plots tend to be 400 - 600 sq yards (call it 500 for the sake of this discussion) with a drive to one side, a smaller front garden and a larger back garden.

The rebuild costs are not wildly different around the country, and there some of them in nearly every postcode sector (there are certainly some in every postcode area, possibly even in every postcode district). So to check whether our land valuations are reasonably accurate, we can use them as a bellwether.

Reality Check #1

Let's assume the rebuild cost is decided to be £80,000 (subject to regional variations in local wages). We then choose a sample of such houses in each postcode sector and look at selling prices. If the selling prices in any area are less than £80,000, the location rent is a negative figure, and so LVT should be reduced, all the way down to a token amount of £1 per square yard if necessary (everybody has to pay something), let's refer to this as Area A, they take no further part in our calculations.

But there is a cut-off area where the LVT is a positive figure (call it £2 per square yard) and the selling price is still above £80,000, that forms our baseline, let's call this Area B. We know that the location rental value, and hence the LVT rate is going to be a larger, positive figure in all other areas (Areas C to Z).

Reality Check #2

If the rental value for the houses in Area B is £5,000 a year and the rent for the nicest one (adjusted for subtle differences in size or build quality) in the nicest area (Area Z) is £20,000, then clearly, the LVT/sq yard in Area Z 'should' be about £30 more/sq yard than in Area B.

So if the LVT rate is £2/sq yd in Area B, it 'should' be £32/sq yard in Area Z, with similar comparisons for every area in between, Areas C to Y.

And semi-detached houses are just one type of bellwether - we can also use:

a) the standard three-bed Victorian terraced house (sub-dividing into those with or without the L-extension at the back; those with or without a front garden, grading for size of back garden etc) and do the same exercise again;

a) concrete ex-council flats built in the 1960s and 1970s (graded by size, number of storeys in block, with or without dbl glazing, central heating, with or without lift), do the exercise again;

c) Barratts 'detached' houses built from the mid-1970s onwards, do it again.

I'd guess that those four broad categories cover about three-quarters of our housing stock, frankly, and if that is not enough evidence to support regional or local variations in the LVT rate in £/sq yard, I don't know what is.
------------------------------------------
And I couldn't give a toss if the individual values are 'subjective', there is such a thing as an objective summary of subjective opinions.

If I were to say "The Rolling Stones are the best pop group ever" then that is my subjective and personal opinion and most people would disagree. I might even be wrong. But if i were to state that I think that they are the best, that is an objective statement.

And if we do an extensive survey and ten per cent of people say they think that The Rolling Stones are the best pop group ever, then it is an objective fact that ten per cent of people think [etc]. And if second most popular choice is ABBA on eight per cent, then that is also an objective fact. Whether The Rolling Stones really are the best pop group ever or ABBA the second best is entirely irrelevant.