We bought a new build in December with Linden Homes in Yorkshire and part exchanged a 4 bed detached with our 3 bed end terrace. We actually got a pretty good deal because, they are only able to offer a certain percentage of the property you wish to buy from them and we worked this in our favour, along with a few other factors.
Our house was on the market at 235k and the house we wanted to buy was 329k but, they advised they wouldn't be able to part exchange at that value because, our home had to be worth no more than 70% of the price of the new home that we wanted to purchase.
We had our house on the market for a while and knew that we would be unlikely to get asking price for it so, we advised we would be willing to 'take a hit' on the price, to make it 70%. However, Linden also dropped the price of the new house to 319,995 in the meantime as, we worked out that it was their year end and they had targets to hit.
We told them we would be willing to accept 224k (70%) for our house, with the added bonus of turf in the garden (it was a big garden so this would have been quite an expense!), tiled flooring in the bathrooms and kitchens and carpets throughout the property. We also, clung on to a passing comment that the sales rep had made regarding 'being able to do us a little something on our stamp duty'. He tried to tell us that we were no longer entitled to this because of their other gestures but, we 'played a good game' (in the sales rep's words) and managed to get them to agree to this also.
After they bought our house, they put it straight back on the market at 235k and it sat there for months and months until, it eventually sold for 220k. We seemed to have got a corker of a deal because, we saved on estate agent fees too by part exchanging.
As others have said, it obviously depends on where you are and the market you are in but, I would say gauge the situation and see if they need you more than you need them. We were told there was someone interested in our plot but, the rep had 'promised it to us first' - He even had a pretend file on his desk that he picked up and said 'Woopsie, I can't let you see that, it's the other buyers details!' However, I am 99% sure no such buyer existed after we, on several occasions, threatened to walk away and they rolled over on our demands pretty much immediately.
Sorry for the essay... Best of luck! x