most developers will do a quick assessment of the difference between the purchase (part exchange price) plus the 'cosmetic surgery' required and the eventual sell price. they will then estimate the time involved in getting the house up to selling point and if financially viable they will do a deal with you. it is usually well in their favour when they do this so if the house is in a bad state, he will get well below market value for the part exchange as the developer will generally want to sell their new build houses before the part ex.