The foundation is the most important — and most variable — part of your garden room project. Here's what the different options mean and how to choose.
Garden Room Foundations: Which Type Do You Need?
The foundation is one of the most important — and most commonly underestimated — aspects of a garden room project. Getting it right is essential for the long-term performance of your building. Getting it wrong can lead to movement, damp, and structural problems.
Why Foundations Matter
A garden room needs a stable, level base that:
- Distributes the weight of the structure evenly
- Prevents movement and settlement over time
- Keeps the floor above ground level to prevent damp
- Can accommodate any slope in your garden
The Main Foundation Types
1. Concrete Slab (Most Common)
A concrete slab is the most traditional and widely used foundation for garden rooms. A layer of hardcore (crushed stone) is compacted, followed by a concrete slab — typically 100–150mm thick.
Pros: Solid, proven, familiar to all builders, good thermal mass
Cons: Permanent, difficult to remove, requires level ground, can crack if ground moves
Cost: £1,500–£4,000 for a typical garden room, depending on size and ground conditions
Best for: Level or gently sloping gardens with stable ground
2. Screw Piles / Ground Screws
Screw piles (also called ground screws or helical piles) are steel screws that are driven into the ground using a machine. A steel frame sits on top of the piles, and the garden room sits on the frame.
Pros: Fast installation (typically 1 day), minimal disruption to the garden, adjustable for slopes, removable
Cons: Higher cost than a concrete slab, requires specialist equipment, not suitable for all ground types (rocky ground can be problematic)
Cost: £2,000–£6,000 depending on size and ground conditions
Best for: Sloped gardens, gardens where minimal disruption is important, or where the garden room may need to be moved in future
3. Timber Frame / Adjustable Feet
Some garden room manufacturers supply their buildings on a timber frame with adjustable feet or pads. The frame is levelled on the ground, and the building sits on top.
Pros: Very fast, minimal groundworks required, can be adjusted
Cons: Not suitable for all ground types, may not be as stable long-term, can be vulnerable to damp if not properly detailed
Cost: Often included in the garden room price or £500–£1,500 extra
Best for: Relatively level, stable ground; temporary or semi-permanent installations
4. Pad Foundations
Concrete pads are poured at regular intervals (typically at corners and intermediate points), with a timber or steel frame spanning between them. Similar to screw piles but using concrete.
Pros: Less concrete than a full slab, can accommodate some slope
Cons: Requires careful levelling, may not be suitable for heavy buildings
Cost: £1,000–£3,000
Best for: Lighter garden rooms on relatively level ground
What Affects Foundation Cost?
The main variables are:
- Slope: A sloped garden requires more work to level — either more concrete or longer screw piles
- Ground conditions: Clay, made ground, or poor drainage increases cost
- Access: Difficult access (narrow side passage, steep garden) increases cost
- Size: Larger buildings need more foundation
Questions to Ask Your Supplier
- What foundation type do you recommend for my site, and why?
- Is groundworks included in your price, or is it extra?
- What happens if ground conditions are worse than expected?
- Can I see examples of your foundation work?
Getting a Site Survey
For any significant garden room project, it's worth having a site survey before committing to a foundation type. A reputable garden room company will visit your site before quoting and factor in the specific conditions.
Foundation requirements vary significantly by site. Always get a site-specific assessment from a qualified professional.