First of all, building work is not cheap at all
My husband is a builder and 10 years ago we put a 4 metre extension to our kitchen which cost us 12K - 4K went underground - we needed to be piloned
Hes doing the same size extension this year - 30/40K for shell, plastering, roofing - electrics and water - comes extra and that's with normal foundations
You need to factor in the costs when applying for your mortgage so you have enough funds to get a loan
Back when we did our first home renovation - complete back of house removed for 6 weeks , things weren't easy, this one - dead easy as we have so many appliances that take little space
We put the kitchen table in the hall, the fridge in one of the downstairs rooms ( spare room in our case ) and on the table I had a microwave, kettle, and George Forman ( would be easier now with an air fryer ) so other then a couple of ping meals , we ate as normal, although washing up was a PITA However if you have dishwasher etc , they can be left plumbed in till the last moment
If the house is perfect in every other way, then an extension makes sense.
My house now is a five bedroom, 3 bathroom, 3 receptions and yet the kitchen/ diner you couldn't fit more then four , now we can seat 12
We also had the garden space to extend without being over looked or encroaching on anyone else's privacy