@Sunsetsupernova We were certainly in a position to ask for incentives and upgrades... and got a resounding no. I really think it's down to each individual development and market conditions at the time. Neither Bloor nor Redrow were offering anything - not even part exchange - on this development because these are big family homes in a very desirable area - often mocked on Mumsnet for it - and they had a queue of buyers.
My previous home was also a new build, by Barratt, and I got turf, fully fitted carpets throughout (including, bizarrely, all three bathrooms) and 5% deposit paid as incentives. But that was in 2001 in a not so salubrious area, and the build was less than six weeks from completion by the time I looked at it.
If properties are flying out the door, then you're not going to get much by way of freebies or other incentives. If they're not, then come the incentives. As I understand it (my BiL is a site manager for one of the other big builders) then if a build gets to the final stages without a buyer then they'll sometimes put upgraded fittings in anyway in the hope of attracting one, and of course if a buyer pulls out after having paid up front for upgrades which have already been fitted and the property goes back on the market, then you'll benefit from that too.
It stands to reason that if a builder is starting a new development and they already have potential customers identified, then the first thing they would do is get those customers in. If you drive past a development every day and notice the sales office opening on a Monday, and by the time you go in on your way home on Thursday they've sold four houses, then that in itself generates interest.
As for the 'process' of buying a new build - it varies between builder and also between the different build stages that you reserve the plot and exchange contracts at, because that will determine the amount of customisation that is possible beyond what the builder actually offers. We wanted £17k or so worth of upgrades/customisation and they wanted 50% up front... fair enough. But the last date at which we could commit to these because of the build progress was before we exchanged contracts, which meant that we had to stump up over £8k at our own risk.
We also found that using the recommended solicitor was extremely helpful, as they had good relationships with Bloor's and could smooth bumps over easily.