These figures could be between £1,200 – 3,000 per m2.
LET’S START WITH A BASIC SINGLE STOREY EXTENSION
SAY 4M X 5M
So, for your 4 x 5m extension, the build cost at the lowest end of these estimates would be roughly £24,000
+ Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer)
Single storey extension = £33,120
DOES IT INCLUDE A KITCHEN OR BATHROOM?
+ Add say, £5,000 for a bathroom. Costs may be more or (if you are very thrifty) slightly less depending on your sanitary ware and finishes.
+ Add £10,000 for a kitchen (again, completely depends on your specification, £10,000 should get you a lower-mid range kitchen with appliances)