Decking Cost Calculator UK (Composite & Timber)

UK Decking Cost Guide 2024

What do I need to know about Material Comparison?

Pressure-treated softwood: cheapest. £25-40/m² for boards. Lasts 10-15 years with regular maintenance (annual oiling). Affordable starter material. Splinters, warps over time. Stained colour fades. Hardwood (iroko, cumaru, garapa, ipe): £70-120/m². Lasts 20-30+ years. Beautiful natural finish. Dense, doesn't splinter as readily as softwood. Sustainably-sourced increasingly available (FSC certified). Composite (basic — wood-plastic composite, WPC): £60-100/m². 15-20 year lifespan. Low maintenance, though basic composites can fade or show wear faster than premium capped composite boards.

What do I need to know about Installation Realities?

Sub-frame is half the project. Treated softwood joists at 400mm centres. Posts/Adjustable footings every 1.5-2m. Joist hangers, galvanised brackets. Galvanised or stainless steel fixings (rust visible on cheap fixings within a season). Cost: £20-40/m² for materials alone. Foundations: ground level: gravel base 100mm + permeable membrane. Low decks: concrete pads or screw piles. Raised decks: deeper concrete footings to frost depth (450mm+). Elevated decks: full structural design needed. Labour: typically 40-60% of the total project cost for a straightforward ground-level deck, rising further for elevated or structurally complex designs.

What should I know about Maintenance Requirements?

Pressure-treated softwood: clean annually (jet wash or brush + decking cleaner). Oil/stain every 1-2 years (£20-40 per deck). Replacement boards every 5-10 years for high-traffic spots. Hardwood: oil with hardwood oil every 1-2 years to maintain colour. If left to grey naturally: clean annually only. Either approach works — depends on aesthetic preference. Composite (WPC): annual clean with deck cleaner. No oiling, sealing, or painting needed. May stain from oily food spills. Recyclable, but only through specialist facilities rather than standard kerbside collection, since composite decking is a mixed material that most recycling centres can't process.

What's the key thing to understand about Planning & Compliance?

Planning permission: usually NOT required for ground-level/low decking under 30cm high covering less than 50% of garden. Required if: deck height over 30cm — many councils require planning even at low heights now. Conservation areas: stricter. Listed buildings: virtually always need consent. Building regulations: required if deck height enables a 1.5m+ drop (railings needed, structural sign-off required). Building Regs Part K: 1.1m minimum balustrade height. 100mm maximum gap between spindles. Check with your local planning authority before building anything over 300mm high, since permitted development rights for decking are more restrictive than for many other garden structures.

What do I need to know about Substructure Costs Hidden in Decking Quotes?

The visible decking boards are often the easiest part of a decking project to price accurately, but the substructure beneath, the joists, bearers, and any posts or footings supporting the deck above ground, frequently makes up a substantial share of the total cost and is where quotes between different suppliers or tradespeople can vary the most. A simple, low-level deck built close to the ground on a firm, level base needs comparatively modest substructure work, while a raised deck built well above ground level, or a deck on sloping or soft ground, needs deeper foundations, posts and additional bracing to remain stable and compliant with good building practice, substantially increasing the substructure cost even though the visible boarded area and finished appearance might look identical to a simpler deck. Composite decking boards, while often priced higher per board than timber, sometimes allow for a lighter or simpler substructure due to the material's different weight and span characteristics, though manufacturer span tables should always guide substructure spacing rather than assuming it matches a timber deck's requirements. Because substructure needs vary so much with height above ground and ground conditions specific to each site, it's one of the main reasons two decking quotes for the same visible deck size and board material can differ significantly in total price.

What should I know about Decking Height and Building Regulations?

How high a deck sits above ground level affects both its construction cost and whether aspects of the build fall under Building Regulations, which is worth understanding before finalising a decking project's budget and design. A low-level deck, generally one where the deck surface sits close to ground level, typically falls outside the scope of Building Regulations in most straightforward garden situations, though local rules and specific site circumstances can vary, so it's worth checking rather than assuming. A deck raised significantly above ground level is more likely to need a handrail or balustrade for safety once it reaches a certain height, both as a sensible safety measure and, depending on height and circumstances, potentially as a Building Regulations requirement, which adds a meaningful cost on top of the decking boards and substructure themselves, as reflected in this calculator's separate balustrade cost input. A raised deck attached to the house, rather than a free-standing structure elsewhere in the garden, may also need to consider how it interacts with damp-proofing at the house wall and door thresholds, since a deck built too high relative to an internal floor level, or without an adequate gap from the house wall, can risk bridging the property's damp-proof course, which is a separate consideration from the deck's basic structural design.

What should I know about Decking Lighting and Electrical Costs?

Deck lighting — post cap lights, recessed deck lights, step lighting, or balustrade-integrated LEDs — adds a meaningful amount to a decking budget beyond the headline board and substructure cost, and it's easy to underestimate because it involves an electrician as well as the decking materials. Low-voltage LED deck lighting kits are the most common DIY-friendly option, using a transformer to step mains voltage down to a safe 12V or 24V before it reaches the deck, and this transformer and cabling need factoring into both material cost and installation time. Any mains-voltage electrical work outdoors, including the circuit feeding the transformer, falls under Part P of the Building Regulations as work in a special location and should be installed or certified by a registered electrician — a cost that's often forgotten when budgeting purely for decking materials and labour. Cabling should be run and protected before boards go down wherever possible, since retrofitting lighting into finished decking means lifting boards, so it's far cheaper to plan and wire lighting in at the substructure stage even if fittings are added later. A simple lighting scheme (a handful of post cap or step lights) typically adds a modest sum to a project, while integrated balustrade or feature lighting can add considerably more.

What do I need to know about Composite Decking Warranty and Lifecycle Cost?

Composite decking typically carries a much longer manufacturer warranty than timber — commonly 20-25 years against structural failure and often 10-30 years against fading, depending on the brand and product tier — reflecting its lower maintenance needs and greater resistance to rot, insect damage, and splintering. When comparing lifecycle cost against timber, it's worth weighing the higher upfront material cost of composite against the near-total absence of ongoing staining, oiling, or sanding that timber decking needs every 1-3 years to stay protected and attractive. Over a 20-25 year horizon, timber decking usually needs replacing or substantially refurbishing at least once, plus regular treatment costs throughout, while a quality composite board, correctly installed with the right substructure and fixings, can realistically last the full warranty period with just occasional cleaning. However, warranties are conditional — most require the manufacturer's specified joist spacing, fixings, and ventilation gaps to have been followed, and improper installation (insufficient ventilation, wrong fixing centres, boards laid too tight) is a common reason warranty claims are refused. Always factor the warranty's conditions into the installation plan, not just its headline length, since an invalidated warranty removes much of composite's lifecycle cost advantage.

Ground-Level vs Raised Decking: What Drives the Cost Difference

Decking cost estimates often assume a simple, low-level deck close to the ground, but raising a deck to clear a slope, step up from a lower garden, or create a level threshold with a raised patio door adds several cost drivers that a flat per-square-metre figure doesn't capture. Footings need to go deeper and be more substantial to support taller posts safely, particularly where the structure needs to resist wind loading and lateral movement at height rather than just carrying downward weight. Posts and bearers typically need to be larger-section timber (or steel) the higher the deck sits, since the same joist spans that work for a 150mm-high deck can become inadequate or require additional bracing once the structure is a metre or more off the ground. A guarding balustrade or handrail is commonly required once a fall from the deck edge reaches a certain height — often cited as around 600mm — and this is frequently the single biggest extra cost on a raised deck, since a compliant balustrade needs its own posts, infill (balusters, glass or cable), and secure fixing back into the deck structure rather than being a simple add-on rail. Access also tends to cost more: a raised deck usually needs a proper step or small staircase down to garden level, built to consistent rise and going dimensions, rather than a single step. Taken together, these factors mean a raised deck of the same floor area as a ground-level deck can cost meaningfully more overall, with the gap widening the higher the deck sits above ground.