Granny Designs

Seven tips on building a garden annexe

In those heady days before Covid, when you could breathe freely over complete strangers, only one in three British employees worked from home.

Thanks to the lockdowns, many thousands now do and, according to a recent YouGov survey, around 60 per cent of them would prefer to continue doing so, either full- or part-time.

Meanwhile, most of us have been spending more time in our homes than ever before, either self-isolating, caring for vulnerable family members or doing our best to keep offspring occupied. Sometimes all three.

Suddenly open-plan layouts no longer seem such a great idea, especially if there are no spare bedrooms or guest rooms ready to be turned into work spaces, or welcome retreats. Slapping on a quick extension or loft conversation or, indeed, moving aren’t really practical options. But, if we’re lucky, there is another, just outside our rear windows.

Garden buildings have become increasingly popular in recent years

Garden buildings have become increasingly popular in recent years - from traditional sheds to dedicated garden offices, log cabins, shepherd’s huts to architect-designed studios, often boasting a full set of gym equipment. Running electricity to them for lighting and heating, installing a wood-burning stove and even adding a water supply and drainage for a sink or a shower are relatively easy.

So surely it would be just as easy to add a bed for the occasional guest and solve your overcrowding problem at a stroke?

The easy answer is: not really, because you are now entering ‘granny annexe’ territory. And there it can be tricky, and occasionally downright hostile.

Here, then, to help you navigate, are seven pointers – with their pluses and minuses.

1 Under permitted development rights you can erect an outbuilding covering up to 50 per cent of your garden space without planning permission.

BUT: Very specific conditions have to be met. The building must be single-storey with a maximum eaves height of 2.5m and a maximum overall height of four metres - for a dual pitched roof - and three metres for a single pitch. It should also lie at least two metres from the boundary. If that’s impossible, the maximum overall height is reduced to 2.5m. The building also can’t be larger than the main house, or be sited forward from the main elevation. Oh, and any veranda or balcony should be no deeper than 0.3 metres.

2 If you intend to use the building as self-contained accommodation, however, planning permission will be necessary. Some local planning authorities have established policies regarding granny annexes, but in general planners tend to be wary, fearing that self-contained accommodation may eventually become a new dwelling that would otherwise be rejected. It’s important, then, to show that the accommodation is ancillary and subordinate to the main house, such as for household members or guests only. It would also be helpful if the only access is via the main house. No suggestion of a separate entrance to the street.

BUT: If you have an existing outbuilding, such as a garage or barn, it can be upgraded to modern accommodation standards without planning permission, though it will still have to comply with building regulations.

3 If planning permission seems problematic, consider applying for a Certificate of Lawfulness under the Caravan Act. This defines both caravans and mobile homes as structures fit for human habitation which are capable of being moved – a definition that can handily apply to garden annexes.

BUT: The Act requires very specific size limitations. Structures should be no more than 20m long and 6.8m wide with an internal ceiling height limit of 3.05m. A modular form of construction, which can be shown to be easily disassembled, would also be more convincing. So brick or stone are definite no-nos.

4 Electrics, water and sewerage must come from the main house. They can’t be separately metered. If possible, site your outbuilding on higher ground than the main house to assist drainage - aim for a fall of at least one in 40. Otherwise, a pump may be needed. Also, check that the local water pressure can cope with the extra distance.

BUT: An off-grid annexe may be possible. If you have a south, or near south-facing garden, you may be able to produce electricity from rooftop photovoltaic panels. A composting toilet or septic tank can deal with sewerage, though you’ll probably still need to carry drinking water from the house.

5 Annexes are liable for council tax.

BUT: If the annexe is being used by a family member or the main house owner, a 50 per cent reduction in the relevant council tax banding can be claimed. If the family member is classed as dependent - ie over 65, substantially or permanently disabled or severely mentally impaired - no council tax is payable.

6 Family members can include elderly relatives, teenagers seeking a degree of independence, ‘boomerang’ offspring returning after leaving home, carers and au pairs.

BUT: Not lodgers. Anyone other than a family member living in a self-contained annexe becomes a tenant, which will contravene planning permission.

7 You can sell a granny annexe.

BUT: Only if it’s disassembled for re-erection elsewhere. As part of your property, however, it will add value.