3 main things I can think of: 1. Poor planning. Get your drawings in order. Make sure they're prepared by a professional and independently peer reviewed if at all possible. Check your calculations. Think they're right? Check them again. Don't try and shortcut the planning system; it never works. Get a Quantity Surveyor to cost up all of the materials. Check your suppliers and don't skimp on quality. 2. Design Changes. Before you start work is the time to review and change anything you're unsure of. So... changed something? You need to check it with the architect, planning dept. QS and structural engineer. Design changes after construction has begun have knock on effects you can't imagine on day one. A change in brick colour might seem innocuous, but your going to feel pretty silly when planning object and get you to take it all down because you didn't get their approval. 3. Bad Project Management. Builders are not, by and large, technical folk. They can read the drawings, but have been known to cut the odd corner if they think what they've done is sufficient. Everyone, not just the client, needs their expectations managed. Work should be hawkishly and regularly checked; plans are there for a reason. Delivery timescales for specialist materials the same, with any delays managed within the existing time budget where possible.