Cost of converting old outhouse

[KurtseyAllslop]

We have a 2mx1.6m outhouse attached to the back of our house adjoining the kitchen. It's single skin brick with a slate roof.

We want to knock through the kitchen wall and make this outhouse part of the kitchen with a small WC at one end. We don't want to take down the whole kitchen wall, just 'cut' a door sized hole IYSWIM.

We also want to bring one of the walls out making it at 2mx2m outhouse. This means diverting a drain slightly

The bricks are fine. It'll need re-pointing, insulating, new floor, new plumbing for WC, plumbing for the boiler and washing machine to be moved out there.

Attached to this outbuilding is also an ugly flat roof contraption which will need dismantling.

Having these very very rough details how much would you guess for a conversion like this?

I've had quotes. I think some are ridiculous. I just want to gauge what you think might be reasonable.

I suspect that if it needs extending, repointing and insulating, it would be cheaper to knock it down and build from scratch. Moving a drain, even slightly will cost money and the whole thing will probably need a build-over waiver from the local water company. My father did something similar a couple of years ago and it cost, from memory, about £4k which seemed enormous to me for such a tiny space.