Do you know the one about actors who don't want to work with children or animals - when it comes to building, the same applies to relatives!!! It's bad enough when a job goes wrong, but when the builder is related, it's a nightmare!!!
The second piece of advice is to have a look around the town or village, and see where other extensions have been done. Ask the owners who did the extensions, and who they would recommend. People are generally very honest, and will only recommend somebody if they did a good job. The local shop proprietor usually will have his or finger on the pulse of the community, and will recommend some few small, general purpose builders.
When you are getting a price, get the builder to set out exactly what they will do, in writing. Compare one with another - you will be surprised what additional things you will pick up in this way.
Ask each builder for one or two comparable reference locations they have done jobs on locally. Visit the houses, and ask the householder what they think of the builder.
If you can afford it, get a decent engineer - one who will do what you want, not what suits him or her. Agree a fee in advance.
Think of practical things - for instance, insist on obtaining a copy of the builder's insurance. If you are purchasing the raw material, have it delivered to your premises. Do not give permission to any builder to purchase stock from the hardware store on your account. Have a formal contract. (Solicitor needed here). Insist on a retainer - settling cracks are all very fine, and will be addressed far faster if there is a retainer.