As a garden room specialist who has built countless structures over the years, I’ve learned one simple truth: a garden room is only ever as good as the base it sits on. No matter how beautifully designed or expertly finished the building is, its long-term performance depends entirely on the strength and accuracy of the foundation beneath it.
There are two main ways to build a garden room base: concrete and timber-framed. Whilst both are suitable options, the concrete base is the stronger of the two. A properly constructed concrete base provides the stability your garden room needs to stay level, the longevity to withstand decades of use, and the moisture protection required to keep the structure dry and problem-free.
In this guide, I’ll walk you through the exact step-by-step process I use on professional installations. You’ll also find practical advice, real-world tips, and worked-out material quantities and cost estimates, so you can build a base that’s as solid and reliable as the room that will stand on it.
Table of Contents
Planning Your Concrete Garden Room Base
Before you even pick up a shovel, planning your garden room base properly is essential. A well-thought-out plan ensures the base is strong enough to last decades, prevents costly mistakes, and helps you accurately calculate materials and costs.
Choosing the Right Location
Start by selecting a flat, well-drained area in your garden. Avoid low spots where water naturally collects, as standing water can undermine even the best-prepared base. Ensure the area is easily accessible for delivery of materials like bulk bags of MOT, cement, and ballast, and consider how you’ll move concrete mixers or wheelbarrows around the site.
Checking Planning and Regulations
While most small garden rooms don’t require planning permission, it’s always worth checking with your local council, especially if your garden room will be near boundaries. You can read the full guide and decide if you need planning permission for your garden room here.
Standard Base Depth
For most garden rooms, a 100–150 mm concrete slab over a 75–150 mm MOT Type 1 sub-base is sufficient. This combination gives a strong, stable foundation capable of supporting the weight of most garden rooms.
Standard Base Layers
The following diagram shows a cross-section of a standard base, including the sub-base, concrete slab, and steel rebar grids. The timber frame shuttering will be removed once the concrete has set.
In some situations, you may need to add a single course of bricks to lift the garden room slightly higher off the ground. This extra height helps prevent water from reaching the timber frame and protects the building from moisture damage.
When building the garden room walls onto the concrete base or bricks, ensure you place a damp-proof course between the timber frame wall.
When a Deeper Base is Required
Certain conditions call for extra depth or reinforcement:
- Soft or clay-heavy ground: If the soil feels spongy underfoot, or you notice pooling after rain, the ground may not support a standard base. In these cases, excavating deeper and adding additional MOT Type 1 layers can prevent the slab from sinking or cracking.
- Uneven or sloped ground: Building a concrete base on sloped ground is usually not recommended because it can be complex and require significantly more materials. In these cases, a timber-framed base is often a better option. However, if the slope is slight, you can achieve a level surface by using stepped or slightly thicker concrete bases.
- Heavy garden rooms: Larger rooms with heavier purposes benefit from a thicker slab to reduce stress and prevent long-term movement.
Measuring and Testing the Ground
- Hand test: Dig a small hole about 300 mm deep and press the soil. Soft, loose, or muddy soil indicates that extra sub-base is needed.
- Drainage check: Fill a hole with water. If it takes more than 24 hours to drain, the soil is poorly draining and a thicker, well-compacted base is recommended.
- Professional assessment: For very large structures or tricky soil types, a professional soil test may be worth investing in.
Marking Out Your Base
Once you know your ground is suitable, measure the footprint of your garden room, adding an extra 75–125 mm around the perimeter for formwork.
Tools & Materials
Below, I’ll list the tools and materials you’ll need. For this example, I use a 100 mm concrete slab over a 100 mm MOT Type 1 sub-base (this is a common specification for good-quality ground).
Tools
- Tape measure, pegs, string line, and marker paint
- Spade, trenching shovel, wheelbarrow, rake
- Plate compactor (vibrating wacker)
- Hammer
- Cement mixer (or access to ready-mix / hire a mixer)
- Float, straight edge (screed board), broom for finish
- Gloves, eye protection, dust mask
Materials
- MOT Type 1 bulk bags (sub-base) (~£80 each) – one bag at 50 mm depth covers 8.5 m².
- Ballast bulk bags (for concrete) (~£75 each)
- Cement – 25 kg bags (~£7.80 each). (We’ll use a 1:5 cement: ballast mix)
- Damp-proof membrane (DPM) (~£18 for a 4x5m sheet)
- Damp-proof course (PDC) (~£6 for a 30m x 112mm roll)
- Reinforcement mesh (A142) (~£25 for a 2.4m x 1.2m mash)
- Timber (for shuttering) (~£20 for a 22mm x 100mm x 4.8m treated timber board).
- Nails/screws.
Where can I find these materials?
Most, or all of these materials can be found at your local building merchants like Travis Perkins.
Step-by-Step Guide: Building the Concrete Base
Step 1: Mark Out the Area
- Use pegs and string to outline the base.
- Alternatively, mark out the area with spray paint.
Step 2: Excavate the Ground
The most labour-intensive part is removing the turf and topsoil. This can be achieved manually using a shovel. Using a mini excavator can significantly help speed up the process.
- Typical excavation depth: 150–200 mm, depending on soil and depth of your base. In our example, the base will have a 100mm sub-base and 100mm concrete. (200mm total)
- As the base needs to be slightly raised from ground level, we should dig down 150mm.
- Remove all vegetation and soft topsoil
Here’s a detailed cross-section of a typical concrete base.
Step 3: Lay and Compact the MOT Type 1 Sub-Base
- Add MOT in layers (50 mm at a time)
- Compact each pass using a vibrating plate
- Use a large spirit level to ensure the sub-base is as level as possible.
- Each bulk bag of MOT will cover roughly 8.5 m² per 50 mm.
Step 4: Install Shuttering (Formwork)
The formwork is made from a simple 22x100mm timber and a set of stakes. This will be used to support the concrete slab whilst it sets in place. The inner edge of the timber frame will be the exact size of your garden room timber frame wall. The shuttering needs to be accurate, level and square.
Check that the shuttering is perfectly level all the way around. This will determine the final height and finish of your concrete slab.
- Use timber boards to create a level perimeter
- Secure with stakes
- Allow extra 75–125 mm outside the building footprint
- Ensure corners are square using the 3-4-5 method
Step 5: Add a Damp-Proof Membrane
Roll out a heavy-duty DPM across the entire base. Overlap all joints by at least 150 mm and tape them securely. Bring the membrane up the inside edges of the shuttering. This keeps moisture from drawing up into the concrete over time.
- Lay DPM across the whole area, overlapping joints
- Tape joins and raise edges up the shuttering
Step 6: Place Reinforcement Mesh
Place an A142 mesh (or similar) on top of small spacers so it sits slightly above the DPM.
This reinforcement helps prevent cracking and adds strength across the entire slab.
For larger bases, overlap mesh sheets by at least one square and tie them together.
- Lift mesh off the DPM using spacers
- Overlap multiple sheets by one square
Step 7: Mix and Pour the Concrete
Mix concrete at a ratio of 1 part cement to 5 parts ballast using a cement mixer. (Hire or purchase). Don’t attempt to mix this by hand on a mixing tray, it’ll take far too long. Work methodically from one corner, pouring and spreading evenly.
Use a straight edge (screed board) to level the concrete along the tops of the shuttering.
- Mix a 1:5 ratio: 1 part cement to 5 parts ballast
- Pour evenly, working from back to front
Step 8: Level and Smooth the Surface
- Use a straight edge (screed board) to level the concrete along the tops of the shuttering.
- Once level, use a float to smooth the surface, or apply a light brush finish for better grip.
Step 9: Cure the Concrete
Concrete needs time to gain strength. For the first 24–48 hours, protect it from rain and direct sun. You can lightly mist it with water or cover it with polythene to prevent rapid drying. Avoid heavy loads for 7 days, and allow the slab to cure for a full 20 days before installing your garden room.
How to Calculate Material Quantities for a Concrete Garden Room Base
Use this simple method for any base size, just swap in your own measurements. I’ve used my 4x5m garden room to show my calculations.
1. Work Out the Area
Multiply length × width.
Example: 4 m × 5 m = 20 m²
2. MOT Type 1 Sub-Base (100 mm total depth)
Supplier coverage: 1 bulk bag covers 8.5 m² at 50 mm
At 100 mm depth, cover = 4.25 m² per bag
Calculation:
20 m² ÷ 4.25 m² per bag = 4.7 → round up to 5 bags
3. Concrete Volume (100 mm slab)
Volume = area × depth
20 m² × 0.1 m = 2.0 m³
Use this volume whether you mix on-site or order ready-mix.
4. Cement & Ballast (1:5 mix)
A 1:5 mix means 1 part cement, 5 parts ballast (6 parts total).
Cement
Cement needed ≈ 20 bags of 25 kg
Ballast
For 2.0 m³ of concrete, most suppliers’ 0.8–1.0 tonne bulk bags will mean you need 3–4 bulk bags of ballast.
(If your supplier lists bag tonnage, divide the required ballast weight by bag size.)
5. Other Materials
- DPM: 1 roll covering 25–30 m²
- DPC: 1 roll of 100-150mm wide
- Reinforcement mesh: 2–3 sheets of A142 mesh
- Shuttering: Timber for an 18 m perimeter (4+5+4+5 m)
- Spacers, tape, screws: 1 small pack of each
Summary for a 4 × 5 m Base
- MOT Type 1: 5 bags
- Concrete: 2.0 m³
- Cement: 20 bags
- Ballast: 3–4 bulk bags
- Mesh, DPM, shuttering: Standard quantities as above
Common Mistakes to Avoid
Even a well-planned garden room base can go wrong if a few key steps are overlooked. Here are the mistakes I see most often, and how to avoid them:
1. Insufficient Excavation
Removing only the turf or top layer isn’t enough. If you don’t dig down to firm ground, the base can settle unevenly, leading to movement in the garden room. Always excavate to the correct depth and check soil quality as you go.
2. Not Compacting the MOT in Layers
MOT Type 1 must be compacted in two 50 mm layers, not poured in at 100 mm. Failing to compact properly results in soft patches that can sink over time. Take your time and compact each lift thoroughly with a plate compactor.
3. Skipping the Damp-Proof Membrane (DPM)
Without a DPM, moisture can rise up through the concrete and into the timber frame, causing long-term damage. A correctly fitted membrane is essential to protect the building from damp and prolong its lifespan.
4. Poor Shuttering That Bulges Under Pressure
Weak or poorly fixed shuttering can bow out when the concrete is poured, leaving you with uneven edges or an oversized slab. Make sure the formwork is straight, well supported, and firmly staked in place before any concrete is mixed.
5. Wrong Concrete Consistency or Poor Finishing
Concrete that’s too wet (high slump) will weaken the slab and take longer to cure. Meanwhile, rushed or uneven finishing can create dips or a rough surface. Aim for a workable but firm mix and take the time to level, smooth, or brush-finish the slab properly.
Conclusion
A strong, well-built concrete base is one of the most reliable ways to support a garden room. When done correctly, it provides excellent stability, long-term durability, and protection against moisture, giving your building a solid foundation for decades of use.
The process is straightforward when broken down into eight clear steps:
- Mark out the area
- Excavate to firm ground
- Lay and compact the MOT
- Fit the shuttering
- Install the DPM
- Position the reinforcement
- Pour and level the concrete
- Allow it to cure properly
That said, a concrete base is not the only option. For those looking for a quicker or more cost-effective solution, a raised timber frame base, as covered in my other article, can also provide a strong, reliable foundation for smaller garden rooms. Can’t decide between a concrete base and a timber-framed base? Check out this guide for the full story.
