If you know from the outset what your budget is, and what you would be prepared to spend, then a professional architect will always try to bring projects in under budget and provide the best value. However, most people want more than can actually achieve with their budget. Unless you tell your architect what your budget is, then there is no way that they can tell you whether your expectations are realistic.
As exciting as your project may be, try to be realistic and honest about the money available. Another point to remember is that generally speaking extensions and alterations are not zero VAT.
Therefore bear in mind that your spending power is reduced by 1/6th of your budget. VAT will be charged on all builder’s fees, materials, building control fees and professional fees. It is commonplace in the building industry for all prices to be quoted nett of VAT.
The good news is that there is no VAT charged on planning application fees, works to adjust a home to suit the requirements of a person with disabilities or works to create a new dwelling. However legislation changes all the time, it is worth consulting HMRC for the latest information.
As a rule of thumb indication of the overall cost of your extension, you can expect to pay approximately £1000 to £1250 per square metre for the building cost. This will of course vary in relation to the quality and complexity of construction.