Anyone had an upstairs extension?

[MsFrog]

Just wondering if anyone has had a bathroom added to their property upstairs, or any addition on the first floor? We are thinking about buying a house but it would realistically need a new bathroom adding above the kitchen. We've had a Google and my husband thinks £15,000 is a reasonable estimate, but I think it would be more. Just wondering if anyone could be me a general figure if they've had something like this done? Thanks in advance.

Is the kitchen a single storey ? Be very careful as this might not have sufficient foundations to hold a second storey best to maybe ask a builder first.

Yes, i have a mid 60s semi and I had an en suite bedroom built above the garage. The kitchen had already been moved into the back of the garage before I had the house.
The building work cost about £15,000. I used two local builders who had done similar extensions on the estate.
It was very straightforward because there was a window at the top of the stairs which was knocked through to make a doorway. Took about four months, slightly longer than planned, but that included Xmas and there were some days when they couldn’t work due to snow or heavy rain.
Altogether with architects fees, planning fees, plumbing, electrics, cost of bathroom suite etc I’d say it came to nearer £25,000.

Thanks for the feedback, we've got a lot to consider

Forgot to say the obvious, I hope it’s added far more on to the value than it cost.