Compare plans and specs. Make sure you are comparing apples to
apples; in many cases the lowest bid is not the best value.
Compare quality and workmanship.
Physically look at contractor's previous work. In most cases,
this will be the deciding factor.
Check for license, there can be a find imposed upon the homeowner
if found guilty of hiring a contractor without a proper license.
Check to see if the contractor is in good standing with local
building department.
Require that the contractor supply you with proof of workman's
compensation. If a contractor does not have it, and one of
his employees are injured on your property, you could be liable.
Will the contractor give any type of structural warranty?
How long has the contractor been in business, will he be there
to honor the warranty, if necessary?
Does the contractor have an experienced workforce that is large
enough to complete the contract in a timely manner?
Every contractor has their own designs, some are better than
others.
Ask for references from other customers.
Be careful of contractors requiring money up front. There
have been cases where contractors have obtained a deposit and never
returned to complete the contract. Payments should be made
only on completed work. The owner should retain a large enough
portion of the contract amount to insure the job's completion. Final
payment should not be paid until work is completed to owner's satisfaction
and the contractor provides a final release of lien.