The average cost of a fully enclosed and insulated garden building or granny annexe in the UK is £18,000 for a building which measures 15 square metres in ground floor area.
How much does a garden building cost?
Although the above is an average garden building cost for the UK, outbuildings of this type do vary significantly in their style, dimensions and specification. Hence, there are a couple of variables which will determine your total garden building cost.
Roof
Most garden buildings feature flat (or single pitch) roofs. If you would prefer a pitched roof, your cost would usually rise by approximately 25% to account for the increase in materials and time to build this style of roof.
Doors
Whilst a modest set of french doors usually costs in the region of £800, it is popular for people to request modern sliding-folding doors which open an entire side (or large percentage) of their proposed garden building as part of the design. A quality installation of 4-panel doors of this type would usually cost in the region of £4,000.
Services
If you require electrical or water services in your new outbuilding, and depending upon the distance between your existing home and your new outbuilding, you should budget approximately £1,000 for installing underground services to your new garden building or granny annexe.
Fixtures and fittings
The cost of all your fixtures and fittings in your outbuilding is entirely ruled by your budget. You can easily spend £10,000 on new luxury fixtures and fittings, or you can spend £500 on cheaper equivalents. Hence, it is hard to provide an average cost for these elements of a garden building or granny annexe.