12 min read · Updated March 2026
On this page
Grounded in stated specification: Pressure-treated C24 structural joists at 400mm centers, concrete support pads, heavy-duty weed membrane, premium capped composite boards with hidden stainless steel expansion clips, fascia trim edging, and waste removal.
How to price thermal expansion gaps, avoid underspecifying joist centers for composite, charge for joist barrier tape, and protect a 20%+ target margin on timber vs composite jobs.
Understand the price difference between pressure-treated softwood and zero-maintenance composite, why cheap composite gets blisteringly hot in summer, and building regulation limits for raised decks.
Decking Cost by Board Type & Size: UK Benchmarks
Material choices dramatically affect total project cost, longevity, and maintenance requirements:
| Deck Area | Treated Softwood | Capped Composite | Installation Duration |
|---|---|---|---|
| 12m² (Compact garden seating) | £1,050 – £1,550 | £1,550 – £2,500 | 1.5 – 2 days (2 crew) |
| 20m² (Standard dining deck) | £1,600 – £2,400 | £2,400 – £4,200 | 2.5 – 3.5 days (2 crew) |
| 35m² (Large split-level deck) | £2,700 – £3,900 | £4,200 – £6,800 | 4 – 5 days (2 crew) |
| 50m² (Major outdoor living area) | £3,800 – £5,500 | £5,900 – £9,800 | 5 – 7 days (2–3 crew) |
Itemised Cost Build-up: 20m² Capped Composite Decking
A deterministic calculation for 20m² of capped composite decking on level ground:
Price your decking project
Tell GoTaskhub what you're doing and get an indicative itemised price based on your exact job dimensions, materials, and ground conditions.
The 3 Mistakes That Ruin Decking Quotes
- Spanning Joists at 500mm Instead of 300–400mm: Softwood can handle 450–500mm joist spans, but composite is more flexible. In hot summer weather, composite boards on 500mm centers will sag and feel spongy underfoot. Composite requires 300mm to maximum 400mm centers.
- Ignoring Thermal Expansion Gaps: Composite boards expand and contract significantly along their length with temperature changes. Installing them tight end-to-end in spring guarantees buckled, warped boards by July. A 5–6mm expansion gap at board ends is mandatory.
- 300mm Building Regulation Rule for Raised Decks: In the UK, any deck higher than 300mm (12 inches) above existing ground level requires full planning permission and building regulations compliance for perimeter safety balustrades.
Related Landscaping Pricing Guides
Compare decking with other garden patio and border options:
- How Much Should a Landscaper Charge? (Day Rates & Pricing Hub)
- How Much Should I Charge for a Patio? (Sandstone & Porcelain Guide)
- How Much Should I Charge for Fencing? (UK Pricing Guide)
- How Much Should I Charge for Turfing? (Lawn Cost Guide)
Ready to turn that estimate into a quote?
GoTaskhub lets you build professional itemised trade quotes in seconds. Include your labour, materials, and terms, then send a secure quote link to your customer to approve online.