Laying a foundation costs €5 to €20 per square metre (materials), depending on the type. For a garden room measuring 3×5 metres (15 m²), a simple pad foundation can be done for €100 to €200. A full concrete slab for the same garden room costs €400 to €800. Hiring someone to do it? Expect to pay roughly double. In this article you’ll find out which foundation type suits your project and how to carry it out step by step.
Which Type of Foundation Do You Need?
1. Pad Foundation (Point Foundation)
Concrete pads at the corner points and in between, with timber beams spanning between them. The most widely used foundation type for garden rooms and sheds.
Suitable for: garden rooms, sheds, canopies, carports, pergolas
Advantages: minimal materials, quick to construct, good resistance to frost heave
Disadvantages: no closed floor — flooring must be installed separately (e.g. timber floor on joists)
Costs: €5 – €10/m² (pads) + timber floor if required
2. Concrete Slab (Full Slab)
A continuous concrete floor slab 8–15 cm thick. The most solid solution for heavier structures.
Suitable for: large sheds, garages, workshops, storage rooms
Advantages: firm, dry floor suitable for all structures
Disadvantages: more material and labour, harder to remove
Costs: €15 – €25/m²
3. Paved Foundation (Slabs on Sand/Gravel)
Concrete paving slabs on a bed of sand or gravel. Not intended for buildings, but suitable for small garden structures, summerhouses or as a base slab for a garden room.
Suitable for: small structures, summerhouses, patio canopies
Advantages: inexpensive and quick to construct
Disadvantages: moves with frost and over time, not suitable for permanent buildings
Costs: €3 – €8/m²
Laying a Pad Foundation: Step by Step
Required Materials (garden room 3×5 m = 15 m²)
| Material | Quantity | Price |
|---|---|---|
| Concrete posts / pad foundations | 6–9 pieces | €30 – €60 |
| Concrete mortar | 100–150 kg | €20 – €30 |
| Levelling board / shuttering | 4 pieces | €20 – €30 |
| Structural timber (beams 68×150 mm) | ±25 metres | €60 – €100 |
| **Total** | **€130 – €220** |
Tools
- Spade
- Hand circular saw
- Spirit level (minimum 80 cm)
- Tape measure
- Builder’s line and profile boards
- Wheelbarrow and bucket
- Rubber mallet
Step 1: Set Out the Area
Mark out the position of the garden room using profile boards and builder’s line. Use the 3-4-5 method to check that the corners are square: measure 3 metres along one side and 4 metres along the other — the diagonal should measure exactly 5 metres.
Step 2: Dig the Foundation Holes
Dig holes at the corners and midway along the long sides (maximum spacing 1.5–2 metres) for the concrete pads. Depth: at least 40–50 cm below ground level (frost-free depth varies by region — check local guidance).
On soft or wet ground: dig deeper or use longer posts (80 cm+).
Step 3: Position the Pads
Fill the base of each hole with 10 cm of gravel for drainage. Place the concrete pads in the holes. Use a spirit level to make sure each pad is vertical.
Fill around the pad with concrete mortar (1:5 ratio of cement to sand). Leave to cure for 48 hours.
Step 4: Level the Tops
After curing: check that all pad tops are at exactly the same height. Use a water level or laser level to verify. Compensate for height differences with plastic shim plates or by grinding down the tops of the pads.
This is the critical step — pads that are not at the same height will result in a wonky garden room.
Step 5: Lay the Beams
Place the structural timber beams (68×150 mm or 63×200 mm) on the pads. Connect the beams to the pads with galvanised joist hangers or anchor bolts. Fit the cross-beam and treat all timber with a wood preservative.
Pouring a Concrete Slab: Step by Step
Required Materials (slab 3×5 m, 10 cm thick)
| Material | Quantity | Price |
|---|---|---|
| Gravel (drainage) | 500 kg | €25 – €40 |
| Insulation boards (PIR 50 mm) | 15 m² | €75 – €120 |
| Reinforcement mesh (steel) | 2 sheets | €40 – €80 |
| Concrete C20/25 (ready-mixed) | 1.5 m³ | €200 – €350 |
| Shuttering (timber boards) | 20 metres | €40 – €60 |
| **Total** | **€380 – €650** |
Step 1: Excavate and Compact
Excavate the area to 35–40 cm below the desired floor level. Compact the base thoroughly. Lay a layer of gravel (10–15 cm) for drainage and compact this as well.
Step 2: Lay the Insulation (Optional but Recommended)
Lay PIR insulation boards (50–80 mm) on the gravel. Insulation prevents moisture and cold rising from the ground. Tape all joints.
Step 3: Install the Shuttering
Place timber boards around the perimeter as shuttering. Check that the shuttering is square and level. Brace the outside with stakes so the shuttering does not bow outwards when the concrete is poured.
Step 4: Install the Reinforcement
Place the reinforcement mesh on spacers (35–40 mm height) so that it sits in the middle of the concrete layer. Tie the sheets together with binding wire.
Step 5: Pour the Concrete
Pour the concrete into the shuttering. Use a vibrating poker to remove air pockets. Strike off the surface level with a straight-edged board. Work systematically from one corner to the other.
Ordering concrete: for small areas you can use ready-mix bags or mix your own (1:2:3 = cement:sand:gravel). For areas over 5 m²: order ready-mixed concrete (€80–€120 per m³ including pump).
Step 6: Curing and Finishing
Protect the concrete from drying out too quickly during the first 24–48 hours (cover with polythene sheeting on warm, dry days). The concrete reaches full strength after 28 days.
Surface treatment: for a smoother, less dusty surface, treat the concrete slab with a concrete sealer or coating (€15–€30 per m²).
Common Mistakes
- Foundation pads not deep enough. Frost heave lifts the foundation — always set pads below the frost line.
- No drainage. Water accumulating under the foundation causes uneven settlement.
- Pads not at the same height. Results in a wonky garden room — always check with a level.
- Reinforcement on the bottom. Reinforcement must be in the middle of the slab, not on the base — use spacers.
- Concrete drying too fast. In summer: dampen the slab on the first day and cover it.
Planning Permission
A foundation for a garden room or shed below the permitted development threshold (max. 150 m², max. 3 metres high, on your own land) is in most cases permitted development. Always check with your local authority for specific rules.
Frequently Asked Questions
How deep does a foundation for a garden room need to be?
Concrete pads should be at least 40–50 cm deep — this takes them below the frost line. In coastal areas sometimes shallower, in higher regions deeper. Check with your local authority if in doubt.
Can I put up a garden room without a foundation?
On firm ground (existing patio, gravel path) a garden room can be positioned temporarily without a foundation. But for a permanent structure, a foundation is always recommended — otherwise the building will settle and twist out of square.
How long does pouring concrete take?
The actual pouring takes 1–2 hours (for 15 m²). Curing takes 48 hours until it can be walked on, and 28 days to reach full load-bearing capacity.
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