Hi James,

Very much depends on the size and type of project, specifics to the site and build etc, however lots of people tend to get builders quotes using only the approved plans and specification. The danger with this is the lack of detail and specifics to your project, and you can end up with significant variance in prices quoted. If you choose a builder this way you also have the risk of paying substantially more than the initial quote, and/or ending up arguing with the builder over costs & item quantities and so on, specifically because of the lack of detail and 'mis-understanding's'.

Programme of Works - sets out what happens when, project duration, key stages, inter-dependencies and so on, and something you would agree with the builder. Basically a Gantt chart, and something a lot of small builders won't use, preferring to quote for example a build schedule of 12-16 weeks depending on the job/weather etc. Always worth using particularly when working with more than one contractor, and when you do use most builders should try to commit to it.

Cost Framework - how the job's being priced; whether via bill of quantities, schedule of rates, fixed price quote, provisional sums for different/specific elements etc. Most builders will price as a fixed lump sum, and will often include in their quote provisional sums for different elements (eg, could be radiators/type, £m3 for extra depth of foundations reqd, specific door furniture etc). For the average extension (if there is such a thing), you should as a minimum aim to get a quote with a Schedule of Works. This lists the specifics of your project and your choices, for everything from windows to sockets & door furniture, includes a lot more detail and should be read/priced against in conjunction with the plans & specification. You can have this drawn up by a project manager/architect or surveyor, and the net result should be that you get more accurate/consistent quotes in and hopefully less surprises down the line (though you should always allow for contingencies, as its very rare that you won't have something extra/unforeseen to pay for).

Site-specific issues, working times etc can be included in the Schedule of Works, as can addressing/highlighting any conditions of Planning.

You can agree a payment plan with the builder, always in arrears, and the specifics of this can be included in a JCT Minor Works Contract, which you can purchase online from RIBA. If you're unsure or don't feel confident dealing with these aspects, you should always get help, and particularly if you ever have any doubts regards payments against valuations/work completed.

Other issues you will have to contend with include the CDM (Construction Design & Management) Regs 2015, and possibly Party Wall matters if applicable, and you can get lots of info about these online. It all sounds a lot scarier than it actually is, and you may feel a bit more confident once you research further. Hope this helps!