Starting with the base, when you should start with the base

Nine times out of ten, problems with a garden cabin can be traced back to the ground beneath it. A cabin is not a shed. By the time you add insulation, glazing, furniture, a sofa bed and a few people, you can easily be putting a tonne and a half or more onto a footprint of a few square metres. A few paving slabs laid on bare soil will not cope with that, and the first sign of trouble is usually a door that stops closing properly.

Do the job properly. That means a suitably sized concrete slab with a damp-proof membrane beneath it, a properly braced timber frame on ground screws, or a paving slab base sitting on a compacted sub-base. As a rule of thumb, you want around 100mm of compacted MOT Type 1, a sharp sand blinding, a membrane, and then your slab or slabs. Ground screws are worth a serious look if your garden slopes or sits on heavy clay — less digging, less spoil to barrow out, and no concrete to cure.

• Get the base level to within a few millimetres across the whole footprint — a cabin cannot be levelled out afterwards with packers.
• Leave at least 300mm of clear air beneath the floor bearers, or raise the cabin on a timber frame, so air can move underneath.
• Never site the cabin in the lowest, wettest corner of the garden just because nothing else grows there.
• Check the manufacturer's base specification before you build anything. Warranties are routinely refused where the base does not match it.

Forgetting to measure the route before you order

This one catches people out on delivery day, when a lorry is already at the kerb. Cabins normally arrive as large panels, and a panel that is 3m long and 2m wide will not negotiate a tight 90-degree turn down a side return, however cheerful the driver is.

Walk the route yourself with a tape measure. Check gate widths, the gap between the house and the fence, low branches, drainpipes, steps, gravel that a trolley will sink into, and where the delivery vehicle will actually stop. A 7.5-tonne lorry needs space to turn around, and some suppliers will not attempt a narrow lane at all. If the panels genuinely will not fit, you have options: a modular cabin built from smaller sections, an on-site build, or a crane or Hiab lift over the house. That last option is perfectly normal, but it costs several hundred pounds and needs booking well ahead.

If the only sensible route runs across a neighbour's land, ask early and get their agreement in writing. It also pays to think about the base, because you will need to get bulk bags, a mixer or a wheelbarrow in before the cabin turns up.

Treating insulation, power and ventilation as afterthoughts

There is a real difference between a summerhouse you use on warm evenings and a garden room you work in every day of the year. If you want year-round use, insulation is not optional: look for insulation between the wall studs, typically 50mm to 100mm of rigid board, plus insulation in the floor and roof. Double glazing with warm-edge spacers makes a noticeable difference to how quickly the space warms up and how well it holds heat.

Plan the electrics properly. That means armoured cable in a trench at least 600mm deep, a dedicated circuit protected by an RCD, and a small consumer unit inside the cabin. Ideally, run the cable before the base is finished, or at the very least before the interior is lined, because retrofitting is far messier and more expensive. Do not be tempted to run an extension lead from the kitchen.

Ventilation is the other half of the equation. Unheated insulated cabins are prone to condensation, and if you plan to cook, shower or dry washing in there, you will need a trickle vent, an extractor or a small dehumidifier. Whatever heating you choose, remember that wood burners are restricted in smoke control areas.

Assuming permitted development covers everything

Many garden cabins do fall under permitted development, but the rules are tighter than people expect. Outbuildings must not be forward of the principal elevation of the house, must be single storey, and normally have a maximum eaves height of 2.5m and an overall height of 4m with a dual-pitched roof (3m for other roof shapes). If the cabin sits within 2m of a boundary, the whole structure generally needs to stay under 2.5m. There is also a limit on how much of the land around the original house can be covered by outbuildings.

Those rights are removed entirely for flats, maisonettes, listed buildings, conservation areas and any property with an Article 4 direction. If the cabin is going to be slept in, or used as a separate dwelling, you will almost certainly need planning permission and building regulations approval. A quick call to your local planning authority, or a certificate of lawfulness, is cheap insurance against an expensive problem later.

Skipping the maintenance that keeps a cabin standing

Timber moves, and a factory base coat is not a finished job. Apply a good-quality wood preserver within a few weeks of installation, once the timber has dried out, and re-coat every 12 to 18 months. Do not seal a damp cabin — you will trap moisture in the wood and encourage exactly the rot you are trying to prevent.

• Clear gutters at least twice a year; overflowing water rots fascias and wall boards quickly.
• Inspect the roof covering annually, whether it is felt, EPDM or shingles.
• Keep a 300mm gravel margin around the base and keep soil, compost and climbing plants away from the walls.
• Pay closest attention to the bottom of the walls and the floor bearers. That is where rot starts.

Buying on price alone, and buying too small

A suspiciously cheap cabin usually means thinner cladding, flimsier glazing, minimal fixings and no insulation. For year-round use, look for cladding around 44mm or more and ask exactly what is included: delivery, installation, floor, roof covering and treatment are sometimes quoted separately.

Size is the other common regret. Wall thickness and insulation eat into the internal footprint, so a cabin advertised as 4m by 3m might give you closer to 3.6m by 2.6m inside. Picture the desk, the chair, the shelving and the person walking between them, then add a fifth again for comfort. It is far easier to buy the right size once than to explain to your family why the garden office has become a very expensive storage box.