AH, this takes me back to first year law school. Contract law.... 3 MUSTS for contract formation.
Agreement on scope
Agreement on time
Agreement on price.
Any one of those missing... no contract formation. Any one of those SUBSTANTIALLY incomplete.... breach of contract. A contract like that can be voided by you, or you can sue for damages. You are required to pay for goods and services provided (to avoid a claim in unjust enrichment) but can find someone else.
Course this is not legal advice, but a mere sharing of knowledge but if your contractor is seriously delinquent (and assuming contract does not have any clauses dealing with lateness, weather, etc) you can tell him or her to take a hike. Just pay for services rendered.
Love the plan for the shop by the way.