In the UK, most garden rooms are able to be built under what's called permitted development. That gives the home owner the right to construct a garden outbuilding without seeking planning permission, as long as it meets several criteria. These are the criteria lifted from the Planning Portal website which apply to houses only:
First things first - what is an outbuilding?
Might sound obvious, but an outbuilding is considered to be a structure that is built for a purpose "incidental to the enjoyment of the dwellinghouse". This could be a garden room, a shed, a greenhouse, garage, kennel, tennis court, swimming pool or any other building that is not connected to your house.
Maximum height - how is this measured?
In England and Wales, the maximum height is measured from the highest part of land adjacent to the garden room. So, if you have a sloping garden and one end of your garden room will be sitting 50mm above the level of the ground at point A, and the other end of you garden room will be sitting 1m above the level of the ground at point B, your maximum height will be measured from the higher of the two, meaning that a building with an overall height of 2.5m (as measured from point A) will effectively be almost 3.5m above the level of the ground at point B.
In Scotland, however, the height is measured from the lowest part of land adjacent to your build. Which would mean, using the same sloping garden example, that a building with an overall height of 2.5m could not go higher than 2.5m above the ground at point B, meaning you'd need to dig out all the ground across to point A to bring that ground level down to match point B. What a pain. (ref: the Scottish Government's Permitted Development Rights, paragraph 4.83)
Now, let's go through those planning criteria above, point by point, for the avoidance of doubt:
No outbuilding on land forward of a wall forming the principal elevation.
This essentially means you cannot build anything past the line of the front of your house (assuming, as in most cases, that the front of your house faces the road or 'highway' - if you have a corner plot you may have two principal elevations) unless you apply for planning permission. As you can imagine it would look a bit odd to stick a building in your front garden, so I would guess that it's unlikely they'd grant permission, even if you were to apply for it.
Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
You can only construct a building of one level.
If you'll be constucting a dual pitched roof - like the typical triangular roof of a house - then the walls can be up to 2.5m high, with the overall height of the building up to 4.0m high.
If you're going for a flat roof then the lower side of the flat roof wall height is still limited to 2.5m, though the higher side can be above 2.5m, as long as the overall height of the building won't exceed 3.0m.
Maximum height of 2.5 metres in the case of a building, enclosure or container within two metres of a boundary of the curtilage of the dwellinghouse.
If your building, once it's finished and clad with whatever materials you are using, will have any part closer than 2.0m to the boundary with your neighbours, then you can scrap what the previous point said about the overall permissible height being up to 4.0m (pitched roof) or 3.0m (flat roof). In this situation in both cases your overall height can now be no more than 2.5m. If you want to go higher then you need to apply for planning permission.
No verandas, balconies or raised platforms (a platform must not exceed 0.3 metres in height).
Essentially you can't surround your garden room with a raised deck or other platform. Having some decking outside is okay, as long as it's no more than 30cm above the ground level.
No more than half the area of land around the "original house" would be covered by additions or other buildings.
The term "original house" means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date). Although you may not have built an extension to the house, a previous owner may have done so, so it's worth checking the deeds, etc.
If you were to measure the square footage of all the land around your house (as it was in July 1948 or as it was when built, if later) - front garden, back garden and at the side/s - then the garden room you're looking to build (plus any other outbuildings you or someone else have already built) must not have a bigger footprint than 50% of that measurement.
In Scotland, the 50% rule applies in reference to the garden area in which your building will be, i.e the back garden only, excluding any hard standing areas, so is less helpful.
In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites the maximum area to be covered by buildings, enclosures, containers and pools more than 20 metres from the house to be limited to 10 square metres.
If you live in any of those four types of area, first of all you must be very lucky because they sound lovely. However, if the garden room you want to build extends past an imaginary line 20m from your house then your garden room can only have a footprint of 10m2 maximum, assuming no other outbuildings are already in place. If no part of your planned build will fall outside of that imaginary 20m line, however, then you can ignore this one.
On designated land buildings, enclosures, containers and pools at the side of properties will require planning permission.
Designated land includes national parks and the Broads, Areas of Outstanding Natural Beauty, conservation areas and World Heritage Sites.
If you live in these areas and want to construct your garden room at the side of your house, instead of in the back garden, then you will need to apply for planning permission first.
Within the curtilage of listed buildings, any outbuilding will require planning permission.
If you live in a listed building then whatever size outbuilding you'd like to construct will require planning permission.
Content above contains public sector information licensed under the Open Government Licence v3.0.
More Info
To read all this in much more detail and to see exactly how it applies to, or differs from, your region of the UK, please look at this post which gives you direct links to all the small print. Please also do contact your local planning department to discuss your project - that's the one guaranteed method you should follow to ensure you keep within the rules.
There's also a nice, simple, visual guide from Planning Portal over on this thread which covers the same points we've looked at above.
There are a few good videos available on YouTube which cover all the above too, including the following:
Outbuildings - do you need planning permission? UK permitted development (England and Wales)
by The Architect
Garden Room Workshop: Part 2. Planning permission and Building regs
by Ali Dymock
