Garden decking is one of the most popular outdoor projects in the UK — and one of the most frequently done poorly. A well-built deck lasts 20 or more years. One built cheaply on a damp sub-frame with untreated timber starts rotting within five. Understanding the right materials, construction method, and planning position before you start makes the difference between a good investment and a problem.
Planning Rules
Decking in most residential gardens falls within permitted development, but with conditions that are often misunderstood:
Decking must not exceed 300mm in height at any point. This is measured from natural ground level, not from the top of any raised area. Decking higher than 300mm — even on a sloped garden — requires planning permission.
The combined area of decking, outbuildings, and extensions must not cover more than 50% of the original garden. Original garden means the garden as it was when the house was built, not as you inherited it.
In designated areas (National Parks, Areas of Outstanding Natural Beauty, conservation areas), decking over 10m² may require planning permission if within 2m of a boundary. Check with your local planning authority before starting.
Timber vs Composite
Softwood (pressure-treated pine or spruce). The most common choice for budget decks. Modern pressure treatment significantly extends lifespan — look for UC4 treatment class for ground-contact timbers and UC3 for above-ground boards. Requires annual oiling or staining to maintain appearance. Lifespan 10–15 years with maintenance, less without.
Hardwood (oak, iroko, balau). Far more durable than softwood. Hardwood decks need little maintenance — occasional oiling to maintain colour, though many homeowners allow them to silver naturally. FSC certification is essential: avoid hardwood from unverified sources. Lifespan 25–40 years. Cost: 2–4x softwood.
Composite decking. Made from wood fibre and recycled plastic. No maintenance beyond washing, doesn't splinter, doesn't rot. Quality varies enormously — cheap composite boards fade, warp, and feel hollow underfoot. Mid-to-upper-range composite (Trex, Millboard, Cladco) performs well and looks good. Costs more upfront but less over 20 years when maintenance is factored in.
Sub-frame and Construction
The sub-frame is more important than the decking boards. A poorly built frame causes movement, squeaking, rot, and eventual structural failure regardless of board quality.
For ground-level decks: concrete pad footings at regular centres (no more than 1.8m), with adjustable steel post bases to keep timber off damp ground. Joists at 400mm centres for softwood boards, 500mm for composite. Cross-bracing if the deck is more than a few metres in any direction.
Boards should be laid with a 5–8mm gap between them for drainage and movement. End-grain sealing at cuts. Fascia boards to finish the perimeter tidily and protect the frame ends from water ingress.
Costs
| Material | Installed cost per m² (2026) |
|---|---|
| Pressure-treated softwood | £90 – £140/m² |
| Hardwood (iroko/balau) | £180 – £280/m² |
| Mid-range composite | £150 – £250/m² |
| Premium composite (Millboard) | £250 – £400/m² |
A 20m² softwood deck including sub-frame, footings, and basic balustrade therefore costs roughly £2,000–£3,500 installed. Steps, lighting, and built-in planters add to this significantly.
Slip Resistance
Wet timber decking is slippery. Anti-slip inserts or grooved boards are not optional on steps and in shaded or north-facing locations where algae builds up. Composite decking with a brushed or embossed surface profile generally outperforms smooth timber in wet conditions. Apply a UV-stabilised decking oil with added anti-slip additive if you're using softwood in a problematic location.