First of all, let's be clear that building regulations are completely separate from planning permission. They're in place to make sure that builders and construction companies build houses and the like to a quality standard, both in terms of materials used and methods employed.
Garden rooms are typically exempt from building regs providing the following criteria applies, here taken from the UK Government publication.
Interpreting this (and ruling out number 2 above on the assumption you are not building an underground nuclear shelter), we can break it down to the following simple questions:
1. Are you going to be using your garden room for someone to sleep in?
Yes: Then you will not be able to escape building regs.
No: Great stuff - move on!
2. Will your garden room have an internal floor area of 15m2 or less?
Yes: Then you are good to go, even if your building is made of wood and is butted right up to your neighbour's fence.
No: Don't panic - move on!
3. Will your garden room have an internal floor area between 15m2 and 30m2?
Yes: If no part of your building, when finished, will be within a distance of less than 1.0m from your property boundary, then you are good to go.
If any part of the building will be within a distance of less than 1.0m from your property boundary (whether it is 99.9cm or 2.0cm) your garden room must be constructed "substantially" of "non-combustible materials" in order to avoid building regs. The reason I put those two things in inverted commas is because they can be interpreted differently between different local council planning departments. The best thing to do is to call your local planning department and ask them how they interpret them. My local planning department told me they'd want the whole of my timber framed building to be clad in a non-combustible material, and that the internal walls should be boarded with fire resistant plasterboard (the pink 'fireboard'). I've heard of other planning departments that only required the wall closest to the boundary to be non-combustible, and others that were fine with a fire-proof paint being applied to a timber exterior.
No: As this means your building's floor area is above 30m2 then that means building regs will apply.
A note about floor area
You may have noted in the questions above that I've referred to internal floor area - this is because that's what matters - not the overall area of your base, but the internal floor area once you've built all your walls.
In all of the above scenarios remember that you must still follow the separate rules for planning permission, covered elsewhere, regardless of whether or not building regs apply.
Ali Dymock put together a great video in 2017 summarising planning permission and building regs - I don't believe anything fundamental has changed since then so it's a good way of reiterating what we've covered above.
Garden rooms are typically exempt from building regs providing the following criteria applies, here taken from the UK Government publication.
Small detached buildings
1. A detached single storey building, having a floor area which does not exceed 30m2, which contains no sleeping accommodation and is a building—
(a)no point of which is less than one metre from the boundary of its curtilage; or
(b)which is constructed substantially of non-combustible material.
2. A detached building designed and intended to shelter people from the effects of nuclear, chemical or conventional weapons, and not used for any other purpose, if—
(a)its floor area does not exceed 30m2; and
(b)the excavation for the building is no closer to any exposed part of another building or structure than a distance equal to the depth of the excavation plus one metre.
3. A detached building, having a floor area which does not exceed 15m2, which contains no sleeping accommodation.
Interpreting this (and ruling out number 2 above on the assumption you are not building an underground nuclear shelter), we can break it down to the following simple questions:
1. Are you going to be using your garden room for someone to sleep in?
Yes: Then you will not be able to escape building regs.
No: Great stuff - move on!
2. Will your garden room have an internal floor area of 15m2 or less?
Yes: Then you are good to go, even if your building is made of wood and is butted right up to your neighbour's fence.
No: Don't panic - move on!
3. Will your garden room have an internal floor area between 15m2 and 30m2?
Yes: If no part of your building, when finished, will be within a distance of less than 1.0m from your property boundary, then you are good to go.
If any part of the building will be within a distance of less than 1.0m from your property boundary (whether it is 99.9cm or 2.0cm) your garden room must be constructed "substantially" of "non-combustible materials" in order to avoid building regs. The reason I put those two things in inverted commas is because they can be interpreted differently between different local council planning departments. The best thing to do is to call your local planning department and ask them how they interpret them. My local planning department told me they'd want the whole of my timber framed building to be clad in a non-combustible material, and that the internal walls should be boarded with fire resistant plasterboard (the pink 'fireboard'). I've heard of other planning departments that only required the wall closest to the boundary to be non-combustible, and others that were fine with a fire-proof paint being applied to a timber exterior.
No: As this means your building's floor area is above 30m2 then that means building regs will apply.
A note about floor area
You may have noted in the questions above that I've referred to internal floor area - this is because that's what matters - not the overall area of your base, but the internal floor area once you've built all your walls.
In all of the above scenarios remember that you must still follow the separate rules for planning permission, covered elsewhere, regardless of whether or not building regs apply.
Ali Dymock put together a great video in 2017 summarising planning permission and building regs - I don't believe anything fundamental has changed since then so it's a good way of reiterating what we've covered above.
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