Building a house or doing addition work is one of the largest single investments a person can make in their lifetime. It's the time for you to do research. Know how much you are willing to spend, how much you can afford, make sure you have the plans in mind for your construction, and know which contractor to pick.
1) Determine how much you can spend. Realize that in all projects, there will be surprise expenses & change orders. Build a buffer into your price.
2) Have specific plans. This is the time to go into all the details. Granite, fixtures, hardwoods, trim, and much, much more. If you don't have good plans, you won't ever know where you'll end up.
3) Find a contractor that you can trust. Research them. Call several other references. Make sure this project is "not over the contractor's head". That's happened to clients in several of my construction litigation cases. Ask your attorney for recommendations of good contractors and find out if your attorney knows of any issues with the contractor.
4) Get a WRITTEN CONTRACT. Spend some time and money with an attorney reviewing the contract to make sure you know and understand the terms. There are many types of construction contracts (fixed costs, costs plus, etc).
5) Make sure you meet with your contractor regularly (at least monthly) to go over the project AND go over the expenses to date and projected costs to finish. You want to know early if you are going over budget.
6) If you make changes, get all CHANGE ORDERS in writing and signed between you and the contractor. Each little change order adds up & the number can be astounding if you let it get out of control.
7) Visit the site regularly. Weekly and sometimes daily.