We did a house extension and loft conversion 4 years ago with new kitchen and bathroom. Had saved up and remortgaged to get the costed price and we had a contingency fund but it all started to go pear shaped from day 1 when we found the foundations needed underpinning and the contingency fund was immediately wiped. We perhaps should have cancelled the loft conversion at that point.
Then the architect made a mistake in the drawings for the loft conversion and we had to get the stairs redone. He footed the bill for the stairs but not for a lot of other stuff that it impacted on like a bespoke shower screen and doors because off the shelf no longer fitted. We were in too far by then to stop.
Some stuff was our fault too. I let the builders talk us into a few extras that we really couldn't afford. I have to say though that now they are things I really love like upstands on the worktops, heated kitchen floor, better quality patio doors.
We also had to unexpectedly replace all the doors in the whole house at great expense right at the end because they didn't meet fire safety standards now that it was a 3 storey building. I still remember the buildings inspector quoting at us 'in the wake of the Grenfell fire' about a loft conversion in a suburban semi!
All in all it was 20% over budget and I had to get an unsecured loan out which fortunately I was able to do. I was devastated at the time to have that extra expense every month for 5 years but in fact we were able to pay it off early after 3 and I am really happy with how much better the house is. I love my new kitchen and bathroom and having a downstairs loo and every time there is a storm I still have to remind myself that the back door will no longer flood like it used to.
It's an investment. Stuff goes wrong but we haven't regretted our overspend in the end because the extension enhances our use of the house every day. I am glad we went ahead even if it was scary at the time.