For anything beyond the smallest job, the normal commonsense rules apply: get three quotes and trust your gut. To make sure you’re comparing like with like, ask everyone to be clear about the exact scope of work: you might find for example that one quote is more expensive because the builder anticipates needing to replaster, whereas someone else has just assumed that the old plaster will be fine. I always insist on a fixed price quote (rather than one based on how long the job takes) so I know in advance how much it will cost, and the builder has the incentive to get the job done and get paid
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