How to Build Your Own Garden Room or Outdoor Space

A proper DIY guide from Wood Green Timber

Whether you want a quiet home office, a gym, a chill-out space or a playroom for the kids, garden rooms are one of the most popular projects in North London right now. And with the right materials (and a bit of graft), they’re well within reach for confident DIYers.

This guide gives you a clear idea of how to build the basic timber-framed shell — the structure that sets the foundation for anything from a simple storage space to a fully finished outdoor retreat.

What You’re Building

This guide covers a timber-framed garden room shell, including:

• Timber frame (walls, floor, roof)

• Sheet material sheathing

• Breathable membrane & battens

• External cladding

• Insulation options

• Door/window openings

You can finish the inside however you like — many people add electrics, plasterboard, or ply lining for use as a home office, gym or studio.

Step 1: Build the Base

Your garden room will need a solid, level base to sit on. There are a few DIY-friendly ways to do this:

• Timber frame base on paving slabs – ideal for small/medium builds

• Landscape ground screws – easy to install and mess-free

• Poured concrete slab – best for heavy, permanent structures (use a pro)

Once your base is sorted, you’ll fix your wall plates and start framing off it.

Step 2: Frame the Walls

Use 47x100mm (4x2) treated timber for wall studs. For larger builds, you can go up to 47x150mm if needed.

• Space studs at 400–600mm centres

• Add horizontal noggins for strength and to support sheathing

• Frame out door and window openings where needed

• Fix everything with structural screws or ring nails

Top tip: Always check your opening sizes against your glazing/door spec before you build — get it right first time.

Step 3: Floor & Ceiling Joists

• Floor: Use 4x2 or 6x2 joists depending on your span and subframe

• Roof: Use 6x2 or 8x2 treated joists for a solid ceiling structure

Lay your floor joists first, fix your sheathing (see next step), then build the walls on top.

Step 4: Sheathing

Board out the walls, floor and roof using OSB or plywood:

• OSB3 is cost-effective and works well for most applications

• Plywood is stronger and has a better finish — but more expensive

Wood Green Top Tip: If you’re insulating internally and covering with plasterboard or cladding, OSB is often all you need — don’t waste money on ply you’ll never see.

Step 5: Insulation

You’ve got two jobs here — thermal comfort and soundproofing.

For walls and ceiling:

• PIR insulation (like Celotex or Ecotherm) is high performance with minimal thickness

• RWA45 slabs or loft roll can be used in combination depending on your budget

For floor:

• Rigid insulation between joists is ideal

• Use foil tape to seal gaps and avoid cold bridging

Step 6: Breathable Membrane & Battens

Wrap the whole structure with a breathable membrane, then fix vertical battens to allow airflow behind your cladding.

This keeps moisture out and helps your cladding last longer.

Step 7: Cladding

Time to make it look the part. We stock a range of smart, modern cladding options:

• Western red cedar TGV – sleek, natural and low maintenance

• Black barn featheredge – a bold, on-trend look

• Treated softwood – a more budget-friendly alternative

Wood Green Top Tip: The back and sides of most garden rooms are hidden by fences — save cash by using standard treated softwood on the unseen sides, and upgrade to cedar or Douglas fir on the front where it matters.

Step 8: Doors & Windows

You’ll need to leave openings for whatever door and window setup you’ve chosen.

Most people opt for:

• Bifold or French doors

• Fixed glazing on the front

• No windows on the back/sides (often hidden)

We don’t stock glazing, but your local supplier will be able to help — just measure carefully and check your sizes before framing!

What You’ll Need from Us:

• Stud timber (C16 or C24) – 47x100mm, 47x150mm, 6x2, 8x2

• OSB or plywood – full range of sheet sizes

• PIR insulation, RWA45, or loft roll

• Breathable membrane & battens

• Cladding – cedar, black featheredge, treated options

• Fixings, postmix, and more

We can help you figure out what you need and how much — just bring in your plan or your sketch and we’ll talk it through.

Thinking About Building One?

We’ve got the materials, advice and delivery service to help you bring it to life — whether you're building something compact or creating your dream backyard retreat.

Pop in and chat to the team — we’ve been doing this since 1981.

Wood Green Timber – real materials, real advice, real people.