My Retreat Garden Rooms · Garden Room Guides

What Is the Best Base for a Garden Room? Concrete, Screw Pile & More Explained

The four main base types, their costs, pros and cons, and which one My Retreat recommends for Surrey gardens.

By My Retreat Garden Rooms  ·  Updated May 2026

The base is the most important part of any garden room build. Get it right and the structure stays level, dry and stable for decades. Get it wrong and you will be dealing with uneven floors, condensation and settlement cracks within a few years. This guide covers every base option, what each one costs, and which one is right for your garden.

Kieron's recommendation

For most Surrey gardens, ground screws are probably the best foundation option. The main reasons are that they are very quick to install and, if there are trees nearby, they avoid severing feeding roots, which can often be an issue with more invasive groundwork. They are also extremely stable because they are installed approximately 1.2 metres into the ground, so if the ground naturally shifts over time, the structure remains very secure.

Ground screws are generally more cost-effective than installing a full concrete raft and they also work very well on sloping or more difficult ground conditions. A concrete raft is worth considering if the garden room is going to house particularly heavy uses, such as a gym with substantial equipment.

I would avoid plastic grid systems or similar lightweight foundations, as ground movement over time can affect the stability and levelling of the garden room.

If ground screws are outside the budget, concrete piles are a good alternative. These typically go around 40 to 50cm into the ground and are more cost-effective, whilst still providing a solid foundation.

Why the Base Matters More Than People Expect

Most customers focus on the design, the cladding and the glazing. The base tends to be an afterthought. This is a mistake. A garden room sits in a garden, exposed to moisture, frost and ground movement year-round. Without a properly engineered base, even the best-built room will move, dip and deteriorate over time.

The base also determines whether your room feels solid underfoot, whether moisture gets in from below, and whether the structure stays level as the ground shifts through the seasons. It is the foundation of everything else, and it deserves the same attention as the rest of the build.

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The Four Main Base Options

Here is a straightforward comparison before the detailed breakdown below.

Base type Approx. cost Best for Rating
Concrete piles £500 – £1,200 Most garden rooms, most ground types Recommended
Ground screws £800 – £2,800 Sloped or difficult ground, trees nearby, faster install Recommended
Concrete raft £1,450 – £5,000+ Heavy equipment, large builds, building regs Situational
Plastic grid / pads £200 – £600 Lighter outbuildings such as sheds only — not suitable for garden rooms Not recommended

Costs above are indicative for garden rooms from 9m² up to 30m² and exclude VAT. Prices were correct at the time of publishing but will vary depending on ground type, waste removal requirements, access and other site-specific factors. They are intended as a guide only. We assess foundations during the free site visit and confirm the actual cost before you commit to anything.

Option 1: Concrete Pile Foundations

Cost: £500 – £1,200  ·  Good alternative if ground screws are outside budget

What it is

Concrete piles are cylindrical columns of concrete poured into bored holes in the ground, typically 200 to 300mm in diameter and 400 to 500mm deep depending on ground conditions. A series of piles support the steel frame floor system of the garden room above, with adjustable fixings to ensure a perfectly level base even on uneven ground.

When it works well

Concrete piles are a cost-effective and reliable foundation where ground screws are outside the budget. They require minimal excavation, cause little disruption to the garden, and provide a solid, permanent base. The steel frame floor system we install on top is one of the things that distinguishes a My Retreat garden room from cheaper alternatives.

Best suited to

Most gardens, most ground types. Works well on level and gently sloping ground. Suitable for all room sizes up to the point where building regulations apply (over 30m2).

Limitations

Not the best option for significantly sloped ground or sites with very poor ground conditions. Your site visit will determine whether piles are appropriate or whether ground screws or a raft are a better fit.

Option 2: Ground Screws

Cost: £800 – £2,800  ·  Kieron's preferred recommendation for most Surrey gardens

What it is

Ground screws are large steel helical piles screwed directly into the ground using a mechanical driver, with no excavation required. They are installed approximately 1.2 metres into the ground. A steel subframe sits on top of the screws, which are individually adjustable to create a perfectly level base regardless of the slope of the ground beneath.

Why it works well

Ground screws are fast to install and cause minimal disruption to the garden. Importantly, where there are trees nearby they avoid severing feeding roots, which can be a real issue with more invasive groundwork. Because they go 1.2 metres into the ground, the structure remains very secure even if the ground naturally shifts over time. They are also fully removable if the room ever needs to be relocated.

Best suited to

Sloped or uneven ground, restricted access sites, gardens where minimal disruption is important. Also a good option where a customer may want to move the room in the future.

Limitations

Not recommended for ground with significant underground obstructions such as old tree roots, rubble or hard rock close to the surface. A ground investigation may be needed on some sites before confirming suitability.

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Option 3: Concrete Raft

Cost: £1,450 – £5,000+  ·  Best for heavy use or building regulations

What it is

A concrete raft is a full reinforced concrete slab poured across the entire footprint of the room, typically 150mm thick with steel reinforcement bar, sitting on a compacted hardcore sub-base. It is the most substantial and most expensive base option.

When it is the right choice

A concrete raft is the best option for a garden room gym with very heavy equipment, particularly if you are installing a full weights rack or heavy machines where point loads on a timber floor would be a concern. It is also required for any build over 30m2, where building regulations apply and the foundation must be designed specifically to suit the soil type.

Best suited to

Gym builds with heavy equipment, rooms over 30m2, sites where building regulations apply, or customers who want the most permanent and solid base possible regardless of use.

Limitations

Higher cost, longer installation time, and significantly more disruption to the garden during groundworks. Concrete is also less environmentally friendly than the alternatives. Not necessary for the majority of standard garden room builds.

Option 4: Plastic Grid Systems and Concrete Pads

Cost: £200 – £600  ·  Limited to small, lightweight structures

What it is

Plastic grid systems use interlocking recycled plastic panels laid on compacted gravel to create a permeable base. Concrete pads are individual precast concrete slabs placed at key points under the structure rather than a continuous foundation.

Where it is appropriate

These systems can work for small, lightweight structures such as a basic summer house or log cabin that will not be used year-round. They are low cost and quick to install.

Why we do not use them for garden rooms

A fully insulated, year-round garden room is a permanent structure with significant weight. Plastic grids and individual pads do not provide the rigidity or permanence needed for this type of build. They are more susceptible to movement, moisture ingress and subsidence over time. My Retreat does not use these systems for our garden room builds.

What My Retreat Uses and Why

Our preferred foundation is ground screws, installed by our specialist partner No More Digging. They are fast, clean, tree-friendly and extremely stable. Where ground screws are not appropriate or outside the customer's budget, we use concrete piles, which remain a solid and cost-effective solution.

We assess ground conditions during the free site visit and confirm the foundation specification before agreeing any price. The cost of the foundation is included in the guide price from our 3D design tool. If your site requires something different, we tell you before you commit.

For gym builds with very heavy equipment, large builds requiring building regulations approval, or complex groundworks including reinforced concrete slabs, we work with our trusted groundworks partner Country Landscapes & Design, who have extensive experience across Surrey and the Home Counties.

Frequently Asked Questions

Does a garden room need a concrete base?
Not necessarily. Concrete piles or ground screws are sufficient for the vast majority of garden room builds and are what most reputable garden room companies use as standard. A full concrete raft is only required for very heavy use, very large rooms (over 30m2), or sites where building regulations specify it. Whether a garden room requires a concrete base depends on the ground type, the size and weight of the structure, and how the room will be used.
How much does a garden room base cost?
The figures below are indicative for garden rooms from 9m² up to 30m² in Surrey and exclude VAT. They were correct at the time of publishing but will vary depending on ground type, waste removal, access and other site-specific factors, so treat them as a guide only. Concrete piles: £500 to £1,200. Ground screws: £800 to £2,800. Full concrete raft: £1,450 to £5,000 or more. At My Retreat, the foundation cost is included in the guide price from our 3D design tool based on a standard build. Any variation is confirmed at the site visit stage before you commit.
What is the best base for a garden room on a slope?
Ground screws are the most effective option on a sloped site. They are individually adjustable so the base can be made perfectly level regardless of how much the ground drops across the footprint. Concrete piles with adjustable fixings can also work on gently sloping ground. A full concrete raft on a slope requires significant groundworks to create a level sub-base, which adds considerably to the cost.
Can I use paving slabs as a base for a garden room?
For a small, lightweight summer house or log cabin, existing paving slabs can sometimes provide an adequate base if they are well-laid, level and in good condition. For a fully insulated, year-round garden room, paving slabs are not adequate. They are not designed to carry the point loads of a permanent structure, they move with frost and ground shift, and they do not provide the level of rigidity a proper garden room requires.
How deep do garden room foundations need to be?
For concrete pile foundations, the depth is typically 400 to 500mm depending on ground conditions. Ground screws are installed to approximately 1.2 metres, which is what gives them their excellent long-term stability even when ground conditions shift over time. A full concrete raft sits on compacted hardcore typically 150mm deep, with the slab itself usually 150mm thick. For builds over 30m2 that require building regulations approval, foundation depth is specified by a structural engineer based on a soil investigation.
Do garden room foundations need building regulations approval?
For garden rooms under 30m2, building regulations do not apply in most cases, which means the foundation does not require formal approval. For rooms over 30m2, building regulations do apply and the foundation must be designed by a structural engineer to suit the soil type. If your garden room will be used as sleeping accommodation, or is attached to the house, building regulations may also apply regardless of size. We advise on this at the design stage.
Are ground screws as good as concrete foundations for a garden room?
Yes, for the right site conditions. Ground screws provide comparable load-bearing capacity to concrete piles, install faster, cause less disruption to the garden, and are fully removable. They are not appropriate for every site, particularly where underground obstructions prevent the screw from reaching the required depth. Your site visit will determine which foundation type is the better fit for your specific garden.

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