If you're doing an extension, why don't you put a bedroom in that (a teenager for example might appreciate being on a different floor from the rest of the family) and make one of the upstairs bedrooms into an extra bathroom? You could even put an extra downstairs loo or bathroom in the extension too? Either as an ensuite to the new bedroom so that the proportions are right, or separately for general use (especially visitors).
On ensuites in general, to each their own.
When I was single I loved an ensuite, so straightforward, especially when sharing a house. Your own bathroom.
Now I'm not single, I don't like them. Principally because I hate hearing someone else going to the loo or anyone else hearing me especially getting woken up in the middle of the night by it, so I prefer a buffer room between my bedroom and the loo. But I also prefer a separate loo and bathroom, as I don't like having to look at a loo whilst reclining in a candle lit bubble bath whilst eating flakey chocolate. And I also think a bath in a big bathroom is better than two small shower rooms, one next to each bedroom.
But I recognise that I am a tiny minority. If you like an ensuite, you like an ensuite.
I think they work best in new builds, where they are properly thought out rooms. When we were househunting last year, I saw a lot of old 2 bed flats that would have had lovely big bedrooms, but both had been cut into to make ensuites, and whilst they could technically still fit a double bed in, they weren't really a double room for two grown ups any more.