It depends on the development and how it is selling. I'm buying a new build and it's a popular site so there wasn't much to be done in the way of money off the price. However I did get estate agent fees, full packing and removal service, legal fees, stamp duty and some additional electric work thrown in.
I think you can try offer between 10% and 15% under the asking price. I was checking on a couple of new builds a few years ago, but did not go ahead. When checking the sold prices recently for the same development, I realized they had sold for approximately, 10% less than asked. They were on the market for between 750k and 894k (3 or 4 beds)
My friend got loads of help, her buyer was going to pull out as it needed damp proof course and roof repairs. Taylor Wilpey paid for the repairs so the sale could proceed nearly 10000 and paid her stamp duty and solicitors and estate agent fees.