You need to define the work you want carried out so that you can ask a builder for a firm price. Your Building Regulations drawing may be enough, especially if supplied by an architect, but it is useful to add terms of payment and info such as how long the work can take and when the builders can be on site. With these 'Contract Documents' you can either ask two or three builders to price it or negotiate with a recommended company.
The Contractors need a few weeks to price the work, they need to check availability and prices for items such as windows and bathroom appliances with manufacturers. Give a date for the return of tenders and avoid an auction situation - forcing down a price will give problems later. Be fair in your selection. Choose on price but be aware how important it will be to get on with your builders. Appoint the contractor in writing with an agreed start date, and ask for a breakdown of the price to aid negotiations later.
There are standard construction industry contracts which pin down the process of tender, work on site and payment, some require an architect to administer them. As with all contracts, they are there to refer to when things go wrong. An architect can minimise the dangers of contractual wrangles through experience with building works, and can quickly settle problems on site by knowing how to work through them and find solutions with the builder while ensuring your interests are maintained.
Try not to change anything from now on, the builder is entitled to charge extra for contract variations, and will need to charge extras for extra work if/when site conditions cause changes. Make sure you all know who is responsible for the site, when materials become yours, and agree insurances for the work and buildings.
You will usually need to pay VAT for alterations, exceptions may be for Listed buildings, disabled access work, and increasingly, for 'green' technology; this must be agreed with your local VAT office in advance.
Bear in mind that there are always unforeseen problems, a good builder is your friend not an enemy, and don't panic. Work 'in the ground' seems to take ages and be affected by weather, rooms which are unplastered, even just unpainted, will look surprisingly small, this will change. finishes take forever. But you will be watching your project slowly become reality. FAQS