Granny Annexe

A granny annexe/granny flat is generally a self-contained unit built in the rear garden which is suitable for an elderly parent.

Does a granny annexe/summer house require planning permission?
If you’re planning to install a bathroom/kitchen in the granny annexe, then you may need to get planning permission. Additionally, local authorities are always mindful that a self-contained building could be a separate dwelling, which would be unlawful and would not achieve planning approval or permitted development.

If the building’s ancillary to the use of the house, then that’s a different story but if the building is going to have its own kitchen and a bathroom, then permission may not be given because the council’s looking to avoid the homeowner potentially renting the building out as a separate dwelling

Without a bathroom/kitchen, you can build quite a generous granny annexe/summer house/home office/pool house or a hobby room under permitted development.

How much does a granny annexe cost?
The main governing factor of the cost of a new building is square footage or square meterage – how big it is. So, the bigger it is, the more it’s going to cost. Typically if you worked out a square meter at £1500, that’s a good base price for basic quality; £1900 per square meter for good quality and £2200 per square meter for excellent quality.

For an average single storey building of 20 square metres, the build cost would be from £30,000 for basic quality, from £38,000 for good quality and from £44,000 for excellent quality.

In addition to the build costs are the interior fit-out costs, which will vary considerably based on the function of the building. Will you be supplying the building with its own water, sewage and electricity and if so, how will your current system need to be modified to cope with the extra demand?

You’ll also need to take into consideration professional, permission and agreement fees depending on your property and what you are planning.

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Additional Costs

  • Professional Fees (builders, architects, tradesmen)
    A new building of 20 square metres at a build cost of £30,000 could potentially attract professional fees from £12,000
  • Permission Fees (planning permission £206 /permitted development £103)
  • Other (insurance, shared drains/walls, building regulations approval or specific environmental/ecological reports for your property)
    All together, it’s worth setting aside at least £2,000 to cover any fees for permissions and sign off

A single storey building of 20 square metres of basic quality, will cost a minimum of £44,000 including most fees, but excluding interior fit-out costs.

To calculate the cost of your project, Yoop Architects take a measured survey of your property, which captures all the information. Then we draw it all up in 3D and create sketches to show sometimes five or six options if it’s not entirely clear how you want to achieve your brief.

A lot of the times there are different ways of achieving your desired result and the more you work with the building, the cheaper, easier and faster your build will be. Once you’ve got a favorite scheme or a couple of favorite schemes, we have a phase called ‘Meet the Builders’. That’s when we can introduce you to a couple of builders, they visit the property and they give you a ballpark figure.


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