With any property purchase, I would strongly recommend getting a bank valuation done first before you put an offer in - you can then decide if the premium the builder is asking is true value or not.
If would also try to find out how keen the builder is to sell - how many appartments are sold / left - ask as many questions as possible.
If you really do like this appartment, don't be afraid to go back in a week or so and try again for a discount. Some buyers are trained to refuse the first 3 offers the get, regardless of the true value / worth.
The reason children are such good negotiators is that they instinctively negotiate without fear and are quite happy to ask for the same thing upto 10 times despite continually being told no; as adults we tend to take rejection as a definite.