You have to be quite brave really! We nearly bought 'off plan' a few years ago but couldn't visualise the environment well enough i.e. proximity to neighbours, road etc based on the plans so we chickened out! Quite glad really as on plans it looked different to the reality when we finally saw it!
Obviously you can get plans from developer and make an offer. You usually have to put down quite a sizable deposit (ours was £1,000). You should be covered by NBCH (I think!) guarantee for any problems once you are in the house. You could ask developer if they have any houses on other developments which are similar to the ones you are interested in to have a look at?