Best Sub Base for a Garden Office Foundation

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Comparison of Sub Base Types for Garden Offices

Base Type Suitable For Pros Limitations
MOT Type 1 + Concrete Slab Permanent, heavy garden offices Highly stable, long-lasting, ideal for year-round use Requires excavation, concrete pour, and curing time
MOT Type 1 + Paving Slabs Medium-weight garden rooms or home studios More DIY-friendly, less expensive than full slab Requires precise levelling, less robust than concrete
Compacted Gravel + Timber Frame Lightweight timber structures or summer houses Quick to install, breathable base, easy to adjust Less suitable for year-round or heavy builds
Ground Screws / Pads Uneven or sloped ground, no-dig installs No excavation needed, minimal ground disturbance May need professional install, not ideal for very soft soil
Screw Pile Foundations Heavier prefab garden buildings Strong, low-impact, works well on clay or slopes Higher cost, often requires specialist tools or hire

Concrete Slab Foundation

  • Extremely strong and stable — ideal for heavy or permanent garden offices.
  • Lasts for decades with minimal movement or settlement if built correctly.
  • Offers a flat, sealed surface that resists moisture and pests.
  • Handles uneven ground better once poured and levelled.
  • Best choice if the building will be insulated and used year-round.

Concrete Slab Foundation

  • Requires more labour, time, and skill — not always DIY-friendly.
  • Higher upfront cost compared to slab or gravel alternatives.
  • Needs proper curing time, which can delay installation.
  • Can be overkill for very lightweight or seasonal garden rooms.

Paving Slab Foundation

  • Faster and easier to install, especially for confident DIYers.
  • Lower cost overall — no concrete pour or curing time required.
  • Decent performance for lightweight to medium garden rooms.
  • Can be adjusted individually to help achieve a level base.
  • Less invasive — good option for tight access or tricky sites.

Paving Slab Foundation

  • Less structural integrity than a full slab — not ideal for heavier buildings.
  • Gaps between slabs may allow moisture, pests, or weeds through over time.
  • More likely to shift or settle slightly without perfect base compaction.
  • Requires careful levelling and alignment to avoid wobble points under the frame.
🔹 Do I really need a sub base for a garden office?
Yes — a proper sub base is essential for keeping your garden office stable, level, and moisture-free. Without one, you risk uneven settlement, water pooling, or long-term structural issues. Whether you’re using a full concrete slab, paving slabs, or ground screws, a solid foundation starts with the right sub base.
🔹 What’s the best sub base material for a garden pod?
For most UK garden pods, MOT Type 1 is the go-to choice. It compacts down hard, drains well, and provides a stable surface for either paving slabs or a poured concrete slab. On sloped or soft ground, you may also consider ground screws or pads to avoid digging and ground disruption.
🔹 How thick should the sub base be for a garden room?
Typically, you’ll want at least 100mm of compacted MOT Type 1 beneath a garden office or pod. For heavier buildings or poor soil, increase this to 150mm. Always compact in layers (around 50mm at a time) to avoid future sinking or shifting.
🔹 Can I build a garden office on paving slabs?
Yes — paving slabs on a properly compacted sub base can work well for small to mid-size garden offices. Just make sure they’re level, laid on a solid mortar bed, and spaced evenly to support the entire frame. For larger, insulated pods, a concrete slab is usually the better long-term option.
🔹 What foundation works best on uneven ground?
If your garden is sloped or patchy, ground screws or adjustable pads are often the best bet. They don’t require heavy digging and can be individually levelled to support your garden pod base. For a concrete or slab base, you’ll need to excavate and level first — which adds time and cost.