Homes in TW15 sell for a median of £465,000 across 431 sales over two years (HM Land Registry), which is 0% against the Spelthorne median of £465,000. Set against the local average rent of £1,661 a month (ONS), that implies a 4.3% gross yield here versus 4.3% for Spelthorne as a whole.
Rents are published per district, not per postcode, so the yield above applies the Spelthorne average rent to TW15 prices. It's the right way to compare postcodes on entry price, but rents do vary street by street, and gross yield ignores costs, voids and mortgage interest.
TW15 averages tell you where to look, not whether one particular flat stacks up. Paste a listing and gemhome runs the actual numbers: yield, cash flow, the lender stress test, EPC work and what the public records say about that specific address.
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