Is the British tax system based on fair market value at all? At least where I live, and I assume in most of the US, taxes are based on the putative value of the property, based on the sale prices of comparable houses in the area.
There is, of course, controversy in determining which homes are "comparable." My house is overvalued, because the "comparable" houses in the neighborhood have all been renovated before they were sold, and I still have the original plumbing, old wiring and no central a/c. I plan to have an independent appraisal or two and appeal the valuation next year.
Do homes with available parking sell for more than homes without? If so, I'd expect that to be built into the appraisal. If not, not. Reading between the lines, and admitting my ignorance of the UK system, it seems that they might just be adding parking access as a factor in which homes are comparable to which -- and if that's the case, I'd expect the valuation of homes without parking to go down (or go up more slowly).
If these changes make the tax valuation more accurately reflect the actual value of the property, they're fair; if not, not. I can't really tell which is the case from this article.
Parking spaces are at a premium in towns - so there is an added value with having that. However I've noticed that the biggest single thing one can have to raise the price of a property is a large garden.
Regards, Ivan