Extension or move?

[Ohhappydays123]

Just need to get it all out and get some opinions please!
DH and I are stuck with this situation:
Moved here 12 years ago - 3 bed semi. Not our forever home. Bought for £250, remaining mortgage of £170k, savings of £200k
We need more space so have been planning a double storey extension to add a kitchen diner, downstairs shower/loo, utility and small double bedroom upstairs. We only plan to stay for another 10 years max to get kids through secondary school and then move further out. We were initially ready to sink whatever money was necessary and double the footprint of the house (at least £330k) but an estate agent said there’s a ceiling price on our road and we wouldn’t get that money back. With extension and extra bedroom we’d get back max £700k but there are no comparables as we’re on an odd plot. House at stands could sell for £500-550.
To move up to a 4 bed in the area would cost at least £600k with work to do on it and £800-900k to get exactly what we want. DH worked out we could probably afford up to £800k at a push but doesn’t really want to extend the mortgage.
We’ve been going back and forth and finally decided we’d go for a smaller extension as our first very rough estimate was £150-200k for that. So we’ve scaled back and had a more detailed quote and it’s coming in at £199k for paint finish but with no floor coverings/kitchen/windows/garden landscaping. So we’re imagining at least another £50k on top to get the finish we want. We would be willing to remortgage to get the extra.
We haven’t approached any other builders yet as we’d been told this guy is very competitive and we know and trust his work so were just going to go with him. We realise now we need to get other builders in to quote but he’s said he’s very confident this is the best price and he’s very knowledgeable and experienced.
We knew it would be expensive - we’re in the south east - just didn’t think it would be THIS expensive. We’ve never done this before as you can probably tell!
Thanks for reading if you’ve made it this far lol - would just be interested to know what people would do in our situation.

I think if you are happy in your road and immediate neighbourhood ie have lovely neighbours then that counts for so much. And if you feel the rest of the house works well for you now then I would seriously think about getting the extension. People often say they may move away but then when the time comes they often stay put, we have!

Thanks! That was my argument to DH. Our neighbours are very old (and deaf, which is helpful with our LOUD DC!) and don’t think they’ll be with us for much longer sadly. Other surrounding neighbours fine. But yes immediate area is good - primary school is walking distance Secondary school 5 mins drive. We have a good network of friends, family between 5 and 20 mins away max.
DH just needs to know how much everything will cost NOW - I’m trying to tell him that some things we’ll just need to wait and see as we go and that actually this price is the reality. But of course it’s our life savings and slightly scary 😬

Would you not go into a loft conversion rather than a two storey extension? And have a simple extension across the back?

Oh good point - should’ve said - not enough space in our loft… and we have a triangular shaped plot of land which has made it all tricky to plan!