Finding out the 'site premium' or 'site-only rental value' is ridiculously easy

I went to Rightmove and searched for 3-bed semi's to rent within a 40-mile radius of Leeds (a circle that includes, Manchester, York, Hull and Sheffield.

You can sort by price - cheapest first or most expensive first.

Comparing visually similar housese, the cheapest one is in Hyde Park, a fairly grotty part of inner Leeds:
The most expensive is this one in Altrincham, a super-posh suburb of Manchester: For sure, the expensive one might be a bit tidier inside (the front garden certainly is), but the bulk of the £1,305 difference is 'site premium'.

You can do similar comparisons for all different categories of homes (different sized flats, terraceds, semi-detacheds, detacheds etc), then do a bit of data smoothing, interpolation and extrapolation and Bob's your uncle.

As far as I can see, the size of a back garden has little impact on rents or selling prices (as opposed to front gardens, which are good for privacy and off-street parking), so drive-by valuations tell you everything you need to know. One person drives and the passenger just puts a tick in the appropriate 'Band' column for each house or door number.