6. Have a contingency. Like other laws of nature, the law of a construction project is that "stuff happens." It could be a problem with the bearing capacity of the soil or uncovering rotted wood when getting ready to build the addition. The best way to deal with the unknown is to allow for a contingency in the budget. The best approach is to start with a higher contingency, say 15% to 20% and then gradually reduce the contingency as you go through the project phases. When you first start the design, you'll have a line item in your budget for a, say, 20% contingency. After the drawings are done and the pieces of the project are identified you might reduce the contingency to 10%. As you you go through construction, you'll be able to reduce the contingency even more so that when construction is complete the contingency is zero.You don't have to spend that contingency. If it isn't used, consider it found money that you can save. That's a great way to feel good about staying on track and coming in under budget.