A garden room, home office or shed is only as good as the base it sits on. A level, well-built concrete base keeps the structure square, dry and stable for decades. Here is how to plan one.
Is a Concrete Base the Right Choice?
Concrete slabs are the most durable option for heavier outbuildings, garden rooms with insulated floors and workshops. Lightweight sheds can sit on other bases, but for anything substantial, concrete is the standard.
How Thick Should the Base Be?
- Small sheds: commonly around 100 mm of concrete on a compacted sub-base.
- Garden rooms and offices: often 100–150 mm, sometimes with mesh reinforcement.
- Workshops or heavy loads: thicker and reinforced, as advised by your installer or engineer.
A compacted hardcore sub-base (often 75–100 mm) and a damp-proof membrane are typically laid beneath the concrete. Many garden room manufacturers specify their own base requirements, so always check them first.
Which Concrete Mix Should You Order?
For domestic bases, mixes around C20–C25 (for example ST2 or GEN3) are commonly used, with stronger mixes for reinforced or heavily loaded slabs. See concrete grades explained, or ask us when you book.
How Much Concrete Do You Need?
Multiply length × width × depth in metres. For example, a 4 m × 3 m base at 0.15 m deep needs 4 × 3 × 0.15 = 1.8 m³. Add about 5–10% for uneven ground. Use our concrete calculator guide.
Getting Concrete Into the Back Garden
Most London gardens have no vehicle access. Options:
- Line pump: pipes run through a side passage or the house, the quickest and cleanest option.
- Wheelbarrows: fine for very small bases, but slow and labour-intensive.
- Volumetric concrete: mixed on site, so you only pay for what the base actually takes.
Step-by-Step Overview
- Mark out and excavate the area.
- Lay and compact the hardcore sub-base.
- Fix level timber formwork around the edges.
- Lay the membrane and any mesh reinforcement.
- Pour, level and finish the concrete.
- Cure and protect it. See how long concrete takes to cure.
Frequently Asked Questions
Do I need planning permission for a garden room base?
Many garden rooms fall under permitted development, but rules vary. Check with your local council or installer.
How long before I can build on the base?
Allow several days at minimum, and longer in cold weather or if your installer specifies.
Planning a home project? See our domestic concrete service.
Need Concrete Delivered in London?
Call 020 8129 0605 or 07723 319888, email sales@metro-ready-mix.com or request a free quote.