Spend more time planning.
No, honestly. While everything is on paper, you can make the choices about where electrical outlets go, what sort of brassware you want, exactly how the windows are going to work, what sort of heating system, which floor finishes you'll be getting and how they will detail up against the walls/old part of the house etc, etc, etc.
Once the project starts, if you haven't got very firm opinions about this stuff and have planned (and costed) it, in some detail, you'll be in the hands of your builder. Which might give you a good outcome, or not, but does tend to generate stress and a lot of unexpected costs.
A good way to check if you have enough plans is to mentally enlarge the plans you have by 10, then look at them and see if the detail on the plans is good enough for someone to actually give you what you want.
Also, don't do what I did with my first building project, and work out a budget which had the following main cost centres:
Building work
Electrician
Plumber
Kitchen units
Bathroom units
Spot the omission? That's right, nothing for decorations, floor finishes, brassware, lighting or curtains. All of which added about another 15% to the costs, IIRC