I know it’s how long is a piece of string all over again, but is 1k per sq m still valid for a rough and ready guess?

Single storey lean-to type extension, no structural changes to main house. Not grand designs, but lots of windows to overlook the garden, patio doors.

And by 1k, I realise that’s likely to mean 1-2k!!

About right I’d say. Generally more in London and home counties, lower as you get further away.

Very much depends on where you are and what spec your going for.
Used to be £1000 approx ten years ago London based

I built my own to a high spec and it cost around £15k for 10sqm. I did everything myself apart from the brickwork. It included a raft foundation which added quite a bit though.

Depends on the size but in the South East it is near 1.5 to 2k. Builders in Brighton are quoting 3 upwards. Generally the big the space the cheaper per sq mt, because there are always some fixed costs

I paid about that. Lots of glass, cut into main roof, underfloor heating and electrics. Plastered but not decorated. Wales (very).