i'm looking to build something with a pitched roof, couple of skylights & patio doors. It will either be one room of around 10*10 feet (dining room/conservatory) or a larger room (18*10) divided into dining room/conservatory plus a utility room. It’d involve ripping out an existing kitchen wall, resiting a boiler & putting in an RSJ. I'm willing to dismantle and dig and decorate as much of it as possible but have no building skills, so it could only be the basics that i do myself (eg. not plastering). I’ve been quoted 40K which made me wince rather a lot.

Most accurate guess gets an invite to dinner and some cake…

Mate on mine just got quoted £27k. Not sure of size but I would guess about 4m X 4m. Thats in Scotland mind.

My folks paid £20 000 for one of similar sort of scale 7 years ago

I live in London and have been quoted from £30K including fitting the kitchen but excluding the units for something very very similar, from the company that did my loft so I know they'd stick to it as they did with the loft.

Another company quoted 45K

Isn't ~£100 sq/ft for the basic building costs about the norm?

£1000.00 per sqm is a good guide.

I have recently finished a 8ft x 12ft brick build with slate roof, two Velux skylights, all solid oak window frames/double doors, exterior wall removed, internal load-bearing wall removed, underpinning and RSJs as required, move electricity meter, convert understairs cupboard to toilet, supply and fit solid oak floor, supply and fit decking and fences. The whole lot came to just under £30k inc. planning, structural engineer, architect fees and all extras such as rewiring/consumer units, plumbing stuff etc.