It's wise to get at least three quotes for your proposed work.
Make sure all quotes are based on the same scope of work and plans, so that they are easily comparable. Using detailed plans and specifications that have building regulations approval will help your builder to quote accurately, avoiding nasty surprises later on.
Remember there is a difference between a quote and an estimate. An estimate is a best guess of how much a job might cost and is often prepared informally with little research. Estimates can change dramatically once work begins if unexpected complications or defects are uncovered, or your scope of work changes. An estimate is not legally binding.
A quote is a more accurate assessment of cost. If you are building an extension, converting a loft or undertaking more involved structural work you should always ask for a quote. To quote accurately, and make sure your project meets building regulations requirements, the contractor may ask for plans, calculations and a scope of work.
Once you've accepted a quote, the contractor has to do the work for that price. However, with an estimate, unforeseen circumstances could lead to extra work, so you should always have additional budget for contingencies and extras.
Remember that the cheapest quote may not be the right one for your project. To make your choice you should always ask for references.