As a rule, you should look to get 3 quotes from reputable builders, quotes can vary by hundreds of pounds in some cases and the cheapest quote is not always the best option either. When you are discussing your project get a feel for the builder, are they able to suggest improvements to the plans? Have they got a big enough team to complete the project in the timescales you want? What has been their previous experience?
Ensure you are also comparing like for like quotes, is painting and decorating included for example? If you don’t understand something don’t be afraid to ask! A good builder will be more than happy to take the time to explain things!