First of all research, research, research. Learn all you can about the contractor you are looking to hire. Finding no bad reviews or information is not any better than finding nothing about him. Look for positive reviews on Angie's List and check with the BBB. Google your contractor to see what else comes up. You'll learn a lot this way. If necessary, ask to see some of his work. He should be able to find at least one or two customers willing to let you visit his current job site or a former one. By doing this you now can have confidence in the contractor you choose.
As for warranty it will vary from state to state. Around here 1 year is pretty typical for most projects. Some states demand longer periods of warranty. Texas has a free reign philosophy allowing general contractors to basically do what they want with no licensing requirements. Read the warranty jargon in the contract carefully. The more convoluted it is the more likely it is they have experience weaseling out of making good on it.
Budget: There are far fewer "surprises" that arise on an addition than a remodel so the bid price should be fairly firm. There are exceptions such as framing issues in the area where the new and old will tie together or the discovery of ground problems where the foundation is built for the new work. I once built an addition on a home that was on a lot which was previously filled in over a natural spring fed pond. Neither I nor the owners knew it until we were almost done and a drought ended. The spring began flowing again and our excavated area for the addition gave it the perfect place to flow. We had to add french drains and dig trenchs to divert the water from the new addition. We later learned about the former state of the land from a neighbor who remembered the lot before the house was built. Obviously this is something I, as the contractor, wasn't responsible for and couldn't have anticipated. I shared the cost with the owners by reducing my charges for the added work but it still added to the cost of the project.
Timeline: Although it may seem like a good idea it isn't wise to put a deadline on your contract. As the deadline approaches the contractor is forced to choose between losing money from the job or cutting corners to get the job done on time. Which do you think he'll choose? A better solution is to have wording to the effect that the work will progress continually through every work day of the week (Monday-Friday) excluding holidays, halting only for incliment weather, natural disaster, or other circumstances beyond the contractor's control. If you've selected the right contractor he's going to make sure things are getting done in a timely manner. The longer it takes, the less money he makes. What you need to be sure of is that there are workers on the site at all expected times. This way you don't get a contractor that starts a job, pauses it to start another, and then returns to finish it later. I personally never take a job with a contracted deadline. The quality of the work comes before the date it's done on the list of priorities. If that means I don't get a job then so be it. I can sleep at night knowing I didn't cheat someone to get the job done faster. At the same time, whenever a job goes beyond my expected timeframe I do go into a bit of a panic mode because every day I have guys working I'm losing profit and potentially losing personal money as well. If I plan for a job to take 2 months and it lasts an extra 2 weeks it is costing me, one way or another. But, I will make sure it's done right above all.
I know you're thinking finding those of us with the same mindset are few and far between. Not really if you spend the time necessary at the beginning of your search. Don't hire someone on a whim. Get to know them. You'll have to "live" with them for the next few months so you'd better be able to get along and trust them. Think back to your college days or your first apartment. Did you let just anyone move in with you or did you get to know them a bit first to make sure they weren't a lunatic? It's kinda the same. You're hiring an individual to do a major project, not buying a tangible item off a shelf where you never have to see the salesman again.
In your contract ask for the cost, payment schedule, details of the work included referencing specific architectural and engineering plans as well as material specifications (all of which you should have before you can collect bids), your responsibilities & liabilities, and the warranty specifics. Whatever the bid is, set aside 10-20% over that for incidentals or improvements to the project if no problems are encountered along the way. Your contractor may do this as well so have an open conversation about this when he presents his contract. Ask him if the cost he has provided includes a little extra for any problems he may encounter. If it does, ask him what will be done with that money if he does not have to use it. Will it be deducted from the final bill or will you be required to upgrade some things to use it up?
Hopefully my lengthy response has given you some valuable information. If it is confusing in any way let me know and I'll gladly elaborate.
Todd's Home Services
San Antonio, TX