Probably more than you think. You can ignore most of the budgets that you see on Grand Designs – these generally only work if you give up your job, manage the construction and do a lot of the building work yourself.
We advise to allow around £2000/m2 for new construction as a good base. Higher levels of performance and specification or more complicated designs will push this higher. As projects get larger there are greater economies to be found as the one off costs are spread over larger areas. The opposite is true for smaller projects.
If you are installing items like kitchens or large areas of high performance glazing these should be considered as stand alone items on top of this. You can get good kitchens from Ikea on a budget, or spend tens of thousands for something hand crafted with top of the range appliances.
You should also factor in a contingency for anything unexpected and around 15% of the construction cost for all of your consultants’ fees and planning and building regs…
Oh, and then there’s VAT. If you are working on an existing house that’s another 20%, and usually 5% for conversions. New build houses are zero rated.
You also need to think about indirect costs too: where will you live while work is happening at your house? Will you need to borrow more on your mortgage to finance the project? Will you need additional insurance while the work is ongoing?
This is why it’s important to plan your project before you start. Work out what you budget is and hone the design to something that is realistic rather than bankrupting. To get a better idea of build costs you can employ a quantity surveyor to prepare a detailed cost plan as the design develops, or get a builder on board early to help hone the detail and the aspirations to the available budget.