ARRRGHHHH. Offer rejected. Plan B

[BatCrapCrazy]

Our maximum offer on what could have been our dream house was rejected. We were planning on completely ripping it out and starting again and putting an extension on.
A house has come up on the next street over and the asking price is our maximum budget.
The house is a bungalow and currently quite small. So we would have to do a dormer extension to put the 3 bedrooms on the top. Has anyone done this and how much did it cost? DP is a builder but doesn't have much experience of dormers so we would have to find someone else to do it.