Northside, Frimley Road, Ash Vale, Surrey – 3 bed rooms (GBP280,000)
https://www.rightmove.co.uk/properties/75614319#/
Extension work 25m2 – Costs GBP19,000 (Basic Quality Works)
https://www.mybuilder.com/pricing-guides/house-extension-costs
Extension value 25m2 – Market value of extension work
https://www.ons.gov.uk/visualisations/dvc434/calculator/index.html
Go to Government website, you can check the extension value after extension work
The extension market value is GBP111,775
The net increase value GBP92,775
The market value of return is 31% (It takes 1 month to get extension approval and 1 month for extension done, together with buy and sell , total time frame is 6 months.)
If this work acts for one year, the return will become 31% * 2 = 62% return