Log Cabin UK: 19mm to 70mm Walls and the 2.5m Height Rule

A log cabin is the most solid way to add a room to your garden without building in brick. In the UK the term covers everything from a small 19mm storage cabin to a 70mm year-round garden annexe, and the wall thickness, the base and the planning rules matter far more than the brochure photos. This page explains what each log thickness is really for, how the planning and building regulations limits apply to a log cabin, and the settling and maintenance points that catch first-time buyers out.

What counts as a log cabin

A garden log cabin is built from solid timber logs, usually spruce or pine, machined with tongue-and-groove faces so they stack and lock together. At the corners the logs are notched so each course interlocks with the one running the other way, the same principle that traditional log buildings have used for centuries. Unlike a framed garden room, the walls are the structure: there are no studs or panels, and there is no insulation inside the wall unless you add it.

Close-up of interlocking notched log corner joints on an old timber building at Oppdal open-air museum, Norway — Traditional notched corner joints on a historic log building at Oppdal museum, Norway. Modern garden cabins use the same interlocking principle with machine-cut logs | Oppdalsmuseet log cabin house corner joints by Wolfmann (CC BY-SA 4.0), via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Oppdalsmuseet_Bygdemuseum_Lafteverk_Laft_hushj%C3%B8rne_Log_cabin_house_corner_joints_Gammelt_t%C3%B8mmer_stokker_Old_weathered_cog-jointed_timber_%C3%85rringer_Graining_Oppdal_Open-air_museum_Norway_2019-04-25_5242.jpg

That matters for two reasons. First, the log thickness sets how warm the building is and what you can sensibly use it for. Second, solid timber moves with the seasons, so a log cabin is built to allow for it.

Log cabin wall thickness: which size do you need?

Wall thickness is the single biggest decision. Tuin, one of the larger UK log cabin suppliers, publishes this guide to what each thickness suits (Tuin guide to log cabin thickness):

19mm: usually only suitable for garden storage.
28mm to 34mm: summerhouses, hobby rooms, playrooms and an occasional home office. Comfortable in spring, summer and autumn; go thicker for winter use.
40mm to 44mm: home offices, garden gyms, studios and hobby rooms used beyond the warm months. A 44mm cabin can be warm in winter with roof and floor insulation, heating and double glazed windows and doors.
58mm: offices, studios and garden rooms used year round, with double glazing and insulation.
70mm: large garden offices and annexe guest accommodation, where Tuin recommends high-quality insulation and double glazing.

Two points from the same guide are worth taking seriously. Insulating the floor and roof gives the biggest improvement in year-round comfort, more than adding wall thickness alone. And thicker logs are heavier, so they need a more substantial base. If you want to work in the cabin through the winter, 44mm is the realistic starting point, with an insulated floor and roof.

Do you need planning permission for a log cabin?

In England a log cabin is treated like any other outbuilding. It usually falls under permitted development, so no planning application is needed, if it meets these limits from the Planning Portal outbuildings guidance:

Not on land forward of the wall forming the principal elevation of the house (usually the front).
Single storey, with eaves no higher than 2.5m.
Overall height no more than 4m with a dual pitched roof, or 3m with any other roof.
No more than 2.5m high if it is within 2m of a boundary.
No verandas, balconies or raised platforms above 0.3m.
Together with other additions and buildings, it must not cover more than half the land around the original house.
On designated land (conservation areas, National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites), buildings at the side of the house need permission.
Any outbuilding within the curtilage of a listed building needs permission.

Permitted development covers houses only, not flats or maisonettes, and it only covers uses incidental to enjoying the house: an office, gym, hobby room or summerhouse. It does not cover use as separate self-contained living accommodation. That is the point where most garden annexe plans need a full planning application, whatever the log thickness.

Watch the 2.5m rule closely. Many cabins are designed to sit just under 2.5m so they can go near a fence, but the height is measured from ground level, so a base raised on a slope can push the total over.

Building regulations for log cabins

Building regulations are a separate test from planning. According to the Planning Portal, a detached outbuilding normally needs no building regulations approval if:

its floor area is under 15 square metres and it contains no sleeping accommodation; or
its floor area is between 15 and 30 square metres, it contains no sleeping accommodation, and it is either at least one metre from any boundary or built substantially of non-combustible materials.

A timber log cabin is not built of non-combustible materials, so a cabin between 15 and 30 square metres needs to sit at least a metre from every boundary to stay exempt. Anything with sleeping accommodation needs building regulations approval, and a new electrical circuit run out to the cabin is notifiable work under Part P, so use a registered electrician who can self-certify it. If you want to sleep in it, read our guide to whether you can sleep in a garden room first.

The base: get it level and square

A log cabin is only as good as its base. Interlocking logs need a base that is level in both directions and square, or the walls will not stack true and the doors will bind. The usual options are a concrete slab, a paving slab base, a timber frame, or ground screws with a timber frame on top. Our comparison of ground screws and concrete bases covers which suits your ground. Remember that thicker logs mean a heavier building, and keep the base slightly smaller than the cabin footprint so rain runs off rather than pooling under the bottom log.

Settling, movement and the gap above the doors

Solid logs absorb moisture in winter and dry out in summer, so the walls rise and drop slightly through the year, especially in the first year. Cabins are designed for it. Windows and doors should not be fixed to the logs, and a small gap is left above each frame, hidden behind the trim, so the logs can move without crushing the frame. Tuin’s own advice is to check that gap periodically through the first year as the seasons change.

Never screw shelves, cupboards or panelling rigidly across several courses of logs; use fixings that allow vertical movement, or fix to one log only.
Do not fill the gap above doors and windows with expanding foam.
Storm braces, where fitted, must be able to slide as the walls move.

Treating and maintaining a log cabin

Many cabins arrive untreated or with only a dip treatment, so plan to treat the timber soon after assembly, inside and out, with a product suited to softwood. Treat all faces, including the bottom log and the end grain at the corners, which soak up the most water. After that:

recoat the outside as the treatment manufacturer advises;
keep gutters clear and plants, soil and wood piles away from the walls;
check the roof covering every year, especially felt;
keep the cabin ventilated and heated through winter if it is a working room, to avoid condensation.

Frequently asked questions

What thickness log cabin do I need for a home office?

For year-round use, 44mm is the realistic minimum, with an insulated floor and roof and double glazing. Tuin recommends 58mm with double glazing and insulation for comfortable all-year use.

Do you need planning permission for a log cabin?

Usually not, if it meets the permitted development limits: behind the front of the house, eaves no higher than 2.5m, no more than 2.5m tall within 2m of a boundary, and covering no more than half the garden. Listed buildings, flats and some designated land need permission.

Can you live in a log cabin in your garden?

Not under permitted development. Using it as separate self-contained living accommodation needs planning permission, and any sleeping accommodation needs building regulations approval.

How close to the fence can a log cabin be?

Within 2m of a boundary the cabin must be no more than 2.5m high. For building regulations, a cabin of 15 to 30 square metres must be at least a metre from the boundary to stay exempt.

Why are there gaps above the doors and windows in my log cabin?

They are deliberate. The logs expand and contract with the seasons, and the gap lets them move without crushing the frames. Do not fill it.

Do log cabins need treating?

Yes. Treat inside and out soon after assembly, including end grain and the bottom log, then recoat the outside as the product directs.

Comparing a log cabin with other options? See more garden room buying advice at Best Garden Room.

Log Cabin UK: 19mm to 70mm Walls and the 2.5m Height Rule

What counts as a log cabin

Log cabin wall thickness: which size do you need?

Do you need planning permission for a log cabin?

Building regulations for log cabins

The base: get it level and square

Settling, movement and the gap above the doors

Treating and maintaining a log cabin

Frequently asked questions

What thickness log cabin do I need for a home office?

Do you need planning permission for a log cabin?

Can you live in a log cabin in your garden?

How close to the fence can a log cabin be?

Why are there gaps above the doors and windows in my log cabin?

Do log cabins need treating?

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