Single Storey Extensions
£1,500 to £2,500 per m² is a realistic estimate for a house extension and reflects price increases over the last few years.
+ Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer).
Two Storey Extensions
As a rule of thumb, you should add 50% extra to the build cost of a single storey extension.
The above figures would apply to a typical uncomplicated extension, assuming a 1.0 meter foundation depth and reasonable access.
Bathrooms
£6,000 - £10,000 would be the general cost for a typical bathroom.
Costs may be more or slightly less, dependent on your sanitary ware, tiling and finish choices.
Kitchens
£5,000 - £10,000 for a kitchen (again, completely depends on your specification, £10,000 should get you a lower-mid range kitchen with appliances).
The above costs assume modest finishes, fittings, electrics and lighting (LED not included).
If you decide on higher-end finishes; e.g. tile finishes, fitted joinery, granite worktops etc then the costs would increase.
Windows/Doors
Standard extension costs are based on Upvc French doors and casement windows, but should you require aluminium bi-fold doors or windows, then costs will increase.