i'm an architect and am quite clued up on pricing jobs of this size, in my area i've done jobs in suffolk which priced at £45k, then done much larger jobs in lincolnshire which priced at £26k, and jobs in cambridgeshire at £35k.

if you sit down with a builder or QS, the daily rates of the workmen vary wildly from county to county.

so you cant just say "a 2-storey side extension will be 40-50k".

also if its a listed building or in a conservation area, or a big old victorian semi / detached the build cost will be alot more than a 80's boring box of a house due to the extra features required etc. and if its on crap ground or near to big trees it will add alot of groundworks cost, as will drainage if it needs re-routing.

i had a builder quote a customer £30k for a job recently, he then got a (rubbish) architect to draw the plans up and he priced it properly at £36k. he then got me to review the plans and cut alot of the un-necessary stuff out of it (not cutting corners because the building had to be flood resilient), and he re-priced it at £22k from my drawings!

so it also depends on what architect has drawn the plans. if its an architect who lives in an ivory tower who has no knowledge of various materials and their costs, and probably never been on site (as is the case with most architects these days), you will get a much much higher build cost from their drawings.