Neither Nationwide nor Zoopla take into account any works done to my property. I assume they are based on sold prices.
The reason we bought this house was because of the outstanding secondary school but the local primary was in special measures. So it was cheap.
Fast forward 8 years and that local primary was rebuilt and is stunning and is now outstanding. So it pushed our house prices up considerably as we are now in catchment for both an outstanding primary and an outstanding secondary.
We have extended our house and added another room. Zoopla is pretty close to what I think my house is worth, Nationwide is 10% below that. But I base my estimation on the sold prices of local houses.