Over-garage extension?

squashy

We are looking to get a top story built onto our garage, in order to gain another bedroom. The garage also has half of the kitchen across the back and so the extension will be right along from the front to the back of the house. It will comprise an access door from the staircase/landing, leading onto a bedroom, study and shower room.

We got a quote for £18500 from our usual builders, but then they let us down as they won a big tender. We got another quote and the bloke took weeks but sent us back a detailed quote (done by a QS!) and it came to £28,000, for the same thing! We got a third quote and that came back at £16,000 but it was not clear what this included and we didn't like him much anyway.

We are keen to go off recommendation only but are really unsure now what would be a fair price to charge. Apparently no underpinning is required as the foundations for the garage/kitchen are deep enough already. We are after a finished job ie plastered and electrics with a basic shower suite.