This is my life right now and tips I have for you.
1. Take the estimated time to complete the project and add a few weeks or even a few months depending on what comes up. There are weather delays, people get sick, people no-show, the wrong/not enough supplies get delivered, and inspections fail.
2. Take the estimated cost to complete the project an add a couple thousand to tens of thousands of dollars. Be prepared for anything especially if you have an older house. There may be pipes that need to be replaced, flooring that needs to be leveled, electrical that needs to be updated, roof decking that needs to be replaced, etc. And then there are things that may be "worth" updating while you are building an addition. Examples include extra insulation, increasing ac unit by a 1/2 ton or more, painting interior/exterior, re-tiling floors, etc.
3. Be prepared for some chaos in your house. A lot of trades start early… like 6 or 7 o’clock early. You may not be able to use certain parts of your house during the construction. You may have to eat or shower at odd times. You may have to sleep in a different place. And dust will be everywhere. You will be sweeping and wiping down counters every-day. Your yard may look like hell due to people parking/dragging things through, or littering on your grass as many trades may be over at the same time.
4. Don’t expect perfection especially since you are adding onto a house. There may be some trouble blending the addition so it looks natural. Also you may be doing some of the work yourself, and may not be highly skilled at certain things like installing baseboards or painting.