- Planning permission: Most decks under 30cm high are permitted development — no permission needed. Confirm with your local authority before starting.
- Best timber for decking: ThermoWood for zero maintenance (25–40 yrs), Siberian larch for naturally durable budget (20–30 yrs), pressure treated softwood for lowest upfront cost (10–15 yrs with annual oiling).
- Joist spacing: Maximum 400mm centres for 26mm boards. Use C16 pressure treated softwood or larch joists — never untreated.
- Fixings: A4 stainless steel throughout — screws or Brofix secret fix clips. Never galvanised.
- Ground clearance: Minimum 50mm air gap below decking boards for ventilation and drainage.
- Cost: £1,400–£2,700 fully installed for a 15m² deck in treated softwood. £2,000–£3,700 in ThermoWood.
Building a timber deck is one of the most impactful garden projects you can undertake — it adds usable outdoor living space, increases property value, and when built correctly will last 25–40 years with minimal ongoing effort. The key decisions — timber species, subframe construction, fixing system, and ground preparation — determine whether your deck looks and performs well in 10 years or needs replacing in 5. This guide takes you through every stage of building a garden deck in the UK in 2026, from planning and timber choice to the final board and finishing coat.
Step 1 — Choose Your Timber
The most important decision in any decking project is the timber species. It determines how long the deck lasts, how much maintenance it requires, and the whole-life cost. There are three main categories for UK garden decking:
| Species | Cost/m² boards | Service life | Maintenance | Best for |
|---|---|---|---|---|
| ThermoWood | £55–£80 | 25–40 yrs | Zero — ever | Premium, contemporary, zero maintenance |
| Siberian Larch | £45–£70 | 20–30 yrs | Minimal — oil optional | Natural timber, good value, low maintenance |
| Pressure Treated Softwood | £15–£30 | 10–15 yrs | Oil annually | Budget builds where maintenance will be kept up |
| Standard Softwood | £10–£20 | 3–8 yrs | Treat every 1–2 yrs or fails | Temporary structures only |
For most UK garden decks — ThermoWood secret fix smooth or brushed 26x140mm is the strongest specification. Zero maintenance, 30-year BRE endorsed life, clean hidden-fix surface, and a whole-life cost that undercuts treated softwood when you factor in annual oiling and replacement. For a naturally durable mid-budget option — Siberian larch. Never specify untreated standard softwood for an exterior deck.
Step 2 — Planning Permission and Building Regulations
Most UK garden decks are permitted development — meaning no planning permission is required — provided all of the following apply:
- The deck is no more than 30cm above ground level at any point
- It does not cover more than 50% of the total garden area (including other outbuildings)
- It is not in front of the principal elevation of the house
- The house is not in a conservation area, national park, or Area of Outstanding Natural Beauty
- The house is not a listed building
If any of these conditions are not met, planning permission is required before work starts. For decks attached to the house (with a ledger board fixed to the wall), building regulations approval may also be required — confirm with your local building control body. See our planning permission guide for more detail on how regulations apply.
Step 3 — Calculate Your Materials
For a standard ground-level deck you will need:
- Decking boards: Measure the deck area in m², add 10% for waste and cutting. Order by m² or linear metres depending on supplier
- Joists: C16 pressure treated softwood 47x100mm or 47x150mm depending on span. At 400mm centres, calculate total linear metres of joist needed
- Outer frame timber: Same section as joists — perimeter of deck plus any intermediate bearers
- Posts: For freestanding decks — 100x100mm or 150x150mm pressure treated hardwood posts, length = deck height above ground + minimum 600mm in ground
- Post bases or concrete: Adjustable steel post bases or 25kg bags of postcrete per post
- Joist hangers: One per joist connection to frame
- Fixings: A4 stainless steel decking screws (4.5x65mm) or Brofix secret fix clips — one clip per joist crossing per board
- Weed membrane: Full area beneath deck plus 300mm margin
- Gravel: 50mm depth under membrane for drainage
Step 4 — Prepare the Ground
Good ground preparation is what separates a deck that stays level and dry for 30 years from one that settles, heaves, and rots from below within a decade. Before any structural work starts:
- Clear all vegetation from the deck footprint — including roots. Any organic matter left beneath the deck will decay and create a damp environment that accelerates structural timber deterioration
- Lay a weed membrane across the full footprint — extend 300mm beyond the outer frame line. Pin at 500mm centres
- Add a 50mm layer of compacted gravel over the membrane — this provides drainage and prevents water pooling directly beneath the structure
- For post-supported decks, dig post holes to minimum 600mm depth — below the UK frost line — to prevent post movement from ground freeze-thaw cycling
Step 5 — Build the Subframe
The subframe is the structural foundation of the deck — get this right and everything above it is straightforward. The subframe consists of the outer perimeter frame, internal joists, and the post or bearer system that carries it.
For a ground-level freestanding deck
Mark out the deck perimeter using string lines and a builder's square. Set the outer frame boards level using adjustable post bases or temporary supports. Check diagonal measurements are equal — the frame must be square before you fix anything permanently.
Fix bearers across the deck at maximum 1.8m centres, resting on post bases or low-level deck feet. Bearers carry the joists and must be level with each other — check with a long spirit level or water level. Use 47x150mm C16 treated softwood for bearers on standard residential decks.
Fix joists to the inner face of the outer frame using joist hangers at 400mm centres. Joists should run perpendicular to the direction the decking boards will run. Check each joist is level — a poorly set joist will show as a high or low board above it. Use 47x100mm C16 treated softwood for spans up to 1.5m.
Before laying any boards, check that the top of the joists is at least 50mm above the gravel/ground below. This ventilation gap is critical — insufficient clearance traps moisture beneath the deck and accelerates joist decay even in treated timber.
For a deck attached to the house
If the deck is attached to the house, a ledger board is fixed to the house wall to support one side of the joist system. The ledger board must be fixed into the structural masonry or timber frame — not just into render or cladding. Install a damp proof course (DPC) strip behind the ledger board to prevent moisture tracking into the wall structure. Flash the top of the ledger with lead or aluminium flashing to shed water away from the wall. For attached decks requiring building regulations approval, the structural engineer or building control officer will specify the ledger fixing specification.
Step 6 — Lay the Decking Boards
Face fix method
Start from the house side (or the most visible edge) and work outward. Place the first board parallel to the house, leaving a 5mm expansion gap between the board and any wall or fascia. Fix with two A4 stainless steel decking screws per joist crossing — pre-drill 5mm pilot holes within 50mm of board ends to prevent splitting. Countersink slightly so screw heads are flush.
Leave consistent 5–6mm gaps between boards — use a spacer or off-cut of the correct thickness to maintain even gaps. Check for level across the boards every 5–6 rows. Continue to the far edge, trimming the final board to width if needed. Snap a chalk line along the outer edge and trim all board ends to a clean straight line with a circular saw.
Brofix secret fix method (ThermoWood grooved boards)
Fix the first board with a Brofix start clip at each joist. For subsequent boards, insert a standard Brofix clip into the groove on the near edge of the new board, position it against the previous board, and drive the clip screw into the joist. The clip simultaneously fixes the board and maintains the correct 5mm gap — no spacer needed. Faster and more consistent than face fixing. Download the full Brofix installation guide for complete specification.
1 — Galvanised fixings: Rust within 2–3 years, staining boards and creating moisture entry at every fixing. Always A4 stainless. 2 — Insufficient air gap: Less than 50mm clearance traps moisture — the #1 cause of premature joist failure. 3 — Untreated joists: Even naturally durable decking boards will survive — but standard untreated softwood joists beneath them will fail within 5–8 years. Always use C16 pressure treated or naturally durable timber for the subframe.
Step 7 — Finishing, Steps and Balustrade
Once all boards are laid, complete the deck with these finishing steps:
- Trim board ends: Snap a chalk line and cut all board ends to a clean straight line with a circular saw. Seal all cut ends immediately with end grain sealer or exterior wood oil
- Fascia boards: Fix fascia boards to close the outer frame and cover the subframe structure. Use the same species as the decking boards for visual consistency
- Steps: Build steps using matching timber — step strings at maximum 500mm centres, treads at the correct riser height (typically 150–175mm)
- Balustrade: Required by building regulations where the deck is more than 600mm above ground. Balustrading must withstand a horizontal force of at least 0.74 kN/m — specify a structural post system accordingly
- First oil coat: If using ThermoWood or larch and you want to maintain colour, apply a UV-stable decking oil such as Koopmans Houtolie before the deck is put into use
Deck Building Costs UK 2026 — Full Breakdown
| Cost element | Treated softwood 15m² | Siberian larch 15m² | ThermoWood 15m² |
|---|---|---|---|
| Decking boards | £225–£450 | £675–£1,050 | £825–£1,200 |
| Subframe timber | £200–£350 | £200–£350 | £200–£350 |
| Fixings, hardware | £80–£150 | £80–£150 | £100–£180 |
| Ground prep materials | £50–£100 | £50–£100 | £50–£100 |
| Total materials | £555–£1,050 | £1,005–£1,650 | £1,175–£1,830 |
| Professional installation | £600–£1,200 | £600–£1,200 | £600–£1,200 |
| Total installed | £1,155–£2,250 | £1,605–£2,850 | £1,775–£3,030 |
| 30-yr maintenance cost | £900–£1,800 (annual oiling + replacement) | £0–£300 (optional oil) | £0 |
For a complete cost breakdown by species see our ThermoWood decking guide and our larch vs ThermoWood decking comparison.
Do I need planning permission to build a deck in the UK?
Most garden decks under 30cm high are permitted development — no planning permission required. Decks over 30cm, covering more than 50% of the garden, or in conservation areas require planning permission. Always confirm with your local authority before starting work.
What is the best timber for decking in the UK?
ThermoWood for zero maintenance and 25–40 year service life at £55–£80/m². Siberian larch for naturally durable mid-budget at £45–£70/m² and 20–30 years. Pressure treated softwood for lowest upfront cost at £15–£30/m² but requires annual oiling and lasts 10–15 years. Never use untreated standard softwood for exterior decking.
How much does it cost to build a deck in the UK?
A 15m² garden deck costs approximately £555–£1,050 in materials for treated softwood, £1,005–£1,650 for Siberian larch, and £1,175–£1,830 for ThermoWood. Add £600–£1,200 for professional installation. Total installed: £1,155–£2,250 (softwood) to £1,775–£3,030 (ThermoWood). Steps, balustrade and lighting add further cost.
What joist spacing do I need for decking?
Maximum 400mm centres for 26mm decking boards. 400–500mm for 28–32mm boards. Up to 600mm for 38mm+ boards. Reduce to 300mm centres in high traffic areas or where heavy loads are expected. Always check the board manufacturer's maximum span recommendations.
Should I use face fix or secret fix decking?
Face fix (visible screws) is simpler — suitable for all decking types. Secret fix (Brofix clips in board groove) produces a completely clean surface with no visible fixings, maintains consistent gaps automatically, and makes individual board replacement easier. For contemporary decks, secret fix is the better choice. Available with ThermoWood grooved profiles.
Decking Timber — All Species from UK Stock
We supply ThermoWood, Siberian larch, pressure treated softwood, and the Brofix secret fix system from UK stock. Tell us your deck dimensions and we'll calculate the precise board quantity, joist timber, fixings and hardware — complete material list with everything you need. Nationwide delivery in 7–14 days.