It would be normal to put the project 'out to tender' at the end of Stage 4 to get comparative quotes. However, if you want to get an idea of the likely cost, or if you are negotiating with one particular builder, the Stage 3 drawings give a pretty complete picture of the intended works and should be sufficient for them to at least give you a rough price. The main information missing at this stage is:
The structural design
Confirmed specifications for floor, wall and roof build-ups
Finishes and fittings (although you may well be able to provide this information yourself)
Speaking to builders at this point is not a bad idea anyway to see how busy they are and to start planning when they ay be able to realistically start the works. A preliminary price will also help to ensure that you are still broadly within your budget and if you are going over you can either consider some value engineering or look to raise further funds.
X
X