Homes in SG9 sell for a median of £455,000 across 195 sales over two years (HM Land Registry), which is +1% against the East Hertfordshire median of £450,000. Set against the local average rent of £1,519 a month (ONS), that implies a 4% gross yield here versus 4.1% for East Hertfordshire as a whole.
Rents are published per district, not per postcode, so the yield above applies the East Hertfordshire average rent to SG9 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.
SG9 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.
Check a buy-to-let