Before work commences, most projects will need Building Regulations approval from Building Control, to ensure design and construction standards are achieved. You should check with your designer before starting work.
For Building Control applications, you will need a set of building regulation drawings and structural calculations. Local Authority Building Control application fees are based on the size and value of works. Your designer or contractor can usually manage the build project for you if it is not something you wish to do yourself. Finding the right builder is essential; choose trades-people based on their previous work and trusted recommendations — not on the cheapest quote. You should also ensure that they have 'contractors all risks insurance'. It is enormously helpful if you manage to find a small group of tradesmen, all of whom have several trades under their wing. This can be crucial in keeping continuity. As a guide, building an extension will typically cost upwards of £1,500 per/m2 + VAT.