We were in a similar situation to the OP. We had a 3 bedroom semi which we had altered over and over again to make the most of the space available. In the end, with a 14yr old daughter and a 18yr old son plus a couple of dogs it became more and more difficult to keep up. It came to a head when a neighbour commented “i dont know how you manage in that little house”. I was gutted tbh.
I looked into extentions above the garage and came to the conclusion that
a) builders are pretty crappy communicators
b) its like rolling dice getting a good one
c) £30k/£40k/£50k take your pick, none of them could give an estimate.
So i had a house worth £150k which was way more than other houses of similar size on estate. It was fully paid for and i could pay for the extention cash with savings. But i would then have a 3 bed semi still (Big bedrooms) worth much much less than the £200k i would have put in it.
We looked around and found a new build house which was double the size for £264k. We were paying a big mortgage payment on the old house to pay it off and we have taken a 25% mortgage on this house so our monthly outgoings are way less than before. Its A rated so heating bills are less and i sold my camper van for double i paid for it so that furnished the entire house.
The biggest plus has been the way family life is. The kids have great big bedrooms all kitted out to their requirements. You can see how happy they are with them. The dining/kitchen is huge and has such a nice open layout with couch, tables, island etc etc. The family tend to sit and chat in there now so the living room literally is for going to for some peace and quiet. Its been lots of manual work for me but so far i havent had a single regret.