Buying guide

Shed base guide: how to prepare the ground for a shed or garden room

Every building we install needs a solid, level base. Here’s what works, how big to make it, and how to have everything ready for installation day.

Why the base matters

A shed or garden room is only as good as the ground it sits on. A level base keeps the frame square, so doors and windows open and close properly for years. It also lifts the timber off wet ground, which helps prevent rot and keeps the floor dry.

Uneven grass, soil or loose gravel will move over time, and that’s when doors start to stick. That’s why we ask for a solid, level base before we install.

Option 1: a concrete slab

A concrete slab is the strongest choice, and it’s what we recommend for garden rooms and larger sheds.

How it’s usually built

Dig out the topsoil and mark out the area.
Lay a layer of hardcore (around 100mm) and compact it well.
Pour the concrete (typically 75mm to 100mm thick) and level it.
Let it cure fully before installation day. Your groundworker will tell you how long to leave it.

Option 2: paving slabs

Well-laid paving slabs make a good base for most garden sheds. The key words are well laid: slabs need to sit on a compacted sub-base with a sand and cement bed, tight together and dead level.

Avoid laying slabs straight onto soil or loose sand. They’ll sink unevenly, and the shed will move with them.

What about grid bases, gravel or decking?

Plastic grid and timber frame bases: these are sold for small sheds. They can work for a light shed on firm, level ground, but they’re not the right choice for a garden room, a chalet or a larger shed. Talk to John before choosing one.
Grass or soil: never build straight onto grass or soil. The ground moves and stays damp, and the floor will rot.
Loose gravel: gravel on its own shifts over time. It’s fine around a base for drainage, but not as the base itself.
Existing decking or patio: it can work if it’s solid, dead level and big enough. Send John a photo and the measurements and he’ll tell you if it’s suitable.

Which base for which building?

Building	Recommended base
Insulated garden room	Concrete slab
Garden chalet with veranda	Concrete slab
Large sheds, 10×10 and up	Concrete slab
Small and medium sheds	Concrete slab or well-laid paving slabs
Bike shed and bin store	Paving slabs or concrete

If you’re planning to add electrics to a garden room later, ask your electrician whether they want a duct laid in the slab before it’s poured. It’s much easier than running a cable afterwards.

How big should the base be?

The base should match the footprint of the building you’re ordering. Shed sizes are given as length x width in feet, so a 10×8 shed needs a base of roughly 10ft x 8ft (about 3m x 2.4m).

Before you pour concrete or lay slabs, call John on 086 733 7925 to confirm the exact base size for the model and size you’ve chosen. Leave room around the base too, so you can walk round the building to treat the timber.

Still deciding on a size? Our shed size guide converts every popular size to metres, and our garden room price guide covers the rooms.

Drainage and where to put it

Avoid low spots. Don’t put the base in a hollow or anywhere water collects after rain.
Keep it above the ground. A base that sits a little higher than the surrounding ground keeps water away from the floor.
Don’t make it too big. A base that sticks out well past the walls lets rainwater sit at the bottom of the timber. Match it to the footprint.
Leave a gap to fences and walls. You need to be able to get round the building to treat the timber, and air flowing round it helps keep it dry.
Think about the door. Make sure the door side faces somewhere you can reach easily with a mower or wheelbarrow.

What does a shed base cost?

The price of a base depends on its size, how easy it is to get to, and how much digging the ground needs. A small paving slab base is a weekend job for a handy DIYer, while a concrete slab for a garden room is usually a job for a groundworker.

When you get quotes, ask what’s included:

Digging out and taking away the soil.
The hardcore layer and compacting.
The concrete, its thickness and finish.
Any steps, ramps or edging.

Get two or three quotes and give each groundworker the exact base size John confirms for your building.

Checklist before installation day

The base is level, solid and fully set.
There’s a clear route from where the van parks to the base, with no locked gates or obstacles.
Plants, pots and garden furniture are moved away from the area.
You’ve told us about steps, narrow side passages or anything else that could affect access.

Not sure if you need planning permission? Read our planning permission guide.

Shed base FAQs

How thick should a concrete shed base be?

Typically 75mm to 100mm of concrete on around 100mm of compacted hardcore. Your groundworker may go thicker for a larger garden room or soft ground.

Should the base be bigger than the shed?

No. The base should match the footprint of the building. If it sticks out too far, rainwater sits against the bottom of the walls. Call John on 086 733 7925 to confirm the exact size before you build it.

Can I put a shed on paving slabs?

Yes, for most garden sheds. The slabs need to sit on a compacted sub-base with a sand and cement bed, laid tight together and level. Don’t lay them straight onto soil.

Does a garden room need a concrete base?

We recommend a concrete slab for every garden room. It’s the strongest option and keeps the room square and level for years.

How long does concrete need before the shed is installed?

It needs to be fully set before installation day. Your groundworker will tell you how long to leave it, which depends on the weather and the mix.