builders can only give a rough estimate as there is alot of variations and types of build and due to not knowing customers exact fixtures and fittings can cause problems. if you've got drawings you could try a quantity surveyor . speak to local builders they can advise people at good prices.

i will be doing my first self build starting march next year, two storey on the side and having priced different builders they seem to price between 500-1500 per square metre, building warrants are based on the actual value of the build so i asked them how to price it and they said average 800 (psqm), so based on floor area of my extension 8m x 4m is 32sqm x 2 for double storey 64 sqm

so prices based on above
builder price between £32000 and £96000
building warrant £51200
these dont add together just a rough guide going from both


scary