Garden Shed — Material & Cost Calculator

Garden Shed Guide

Permitted Development — When Do You Need Planning?

In England, most garden sheds are permitted development (no planning permission needed) if: single storey only. Maximum height: 2.5m within 2m of a boundary. Maximum height: 4m for a dual-pitched roof OR 3m for any other roof, if more than 2m from boundary. Does not extend beyond the side of the house (outbuilding only, not side extension). Permitted development does NOT apply: on listed buildings or in conservation areas (always check). To buildings forward of the principal elevation of the house facing a road, which generally do need planning permission regardless of size.

What's the key thing to understand about Base Construction?

Concrete slab: most durable. Depth: 100mm minimum (150mm for larger sheds). Mix: 1 part cement : 2 parts sharp sand : 3 parts 20mm aggregate. Or use C25 ready-mix for larger pours. Reinforcement: A142 mesh for larger slabs. Paving slabs on hardcore: 50mm MOT Type 1 hardcore, compacted. Then 25mm sand bed. Then slabs. Adequate drainage essential — do not trap water under the shed. Timber bearers: 100×75mm treated timber on 100mm shingle/gravel. Allows ventilation under floor — prevents rot. Cheapest base option for a small, lightweight shed, though less durable long-term than a concrete slab.

What do I need to know about Timber Specification?

Framing: C16 grade treated timber (47×100mm or 47×75mm studs at 600mm centres). Cladding thickness: minimum 12mm for shiplap. 16mm for better insulation and rigidity. Roof: OSB/3 18mm minimum for roof decking. Then 3G underlay felt. Then mineral surface felt strips. Or corrugated bitumen/metal roofing. Treating timber: all structural timber should be pressure-treated for in-ground or near-ground use. External cladding: paint or stain annually with exterior-grade wood preserver. Floor: 12mm exterior-grade plywood or OSB is the minimum sensible thickness to avoid flexing and sagging under weight over time.

What do I need to know about Cost Estimates?

DIY timber shed costs (2024): small shed (1.8×2.4m): materials £300-600. Medium shed (2.4×3.0m): materials £600-1,200. Large shed/workshop (3×5m): materials £1,500-3,000. Supply-only flat pack shed (similar sizes): £300-800 (thin cladding, limited lifespan). Purpose-built quality shed: £1,200-3,500 erected. Insulating for year-round use: add 50mm mineral wool between studs + vapour barrier + internal lining board. Additional £300-600 for insulation. Key trade-off: a quality DIY build with 16mm cladding and a proper base costs more upfront than a cheap flat-pack shed, but lasts significantly longer and needs far less maintenance.

What's the key thing to understand about Choosing the Right Shed Material?

The three most common garden shed materials — timber, metal, and plastic/resin — each suit different priorities. Timber sheds offer the most traditional appearance and tend to be the easiest to customise (adding shelving, insulation, or windows), but require more ongoing maintenance, typically needing to be treated with preservative or paint every one to two years to prevent rot and weather damage, and are generally the most expensive option for a comparable size. Metal sheds are usually the cheapest and most resistant to rot and pests, but can be prone to condensation inside (since metal doesn't breathe or insulate the way timber does, moisture can build up on the internal walls and ceiling), can dent, and tend to feel less secure against determined forced entry at the cheaper end of the range despite often including a lock. Plastic or resin sheds require essentially no maintenance, won't rot, rust, or need repainting, and resist fading reasonably well, but typically have less structural rigidity than timber or metal for the same price and can suffer in extreme temperature swings. Base type matters as much as the shed material itself: a solid, level, well-drained base (concrete, paving slabs, or a purpose-built plastic shed base) is essential regardless of shed material, since a shed placed on an uneven or poorly draining base will develop problems with moisture, pests, or structural warping regardless of how good the shed itself is.

What should I know about Shed Size Planning and Legal Permitted Development Limits?

In the UK, garden sheds and other outbuildings can usually be built without planning permission under 'permitted development' rules, but only within certain limits — commonly restrictions on how close to a boundary a shed with an eaves height above a certain threshold can be placed, a maximum height limit (lower if within a set distance of a boundary), and a limit on the total area of ground that outbuildings can cover relative to the garden's original size, all of which can vary for listed buildings, conservation areas, or flats, where permitted development rights are more restricted. Checking current local authority guidance before building a larger shed, rather than assuming permitted development automatically applies, avoids the risk of being required to modify or remove a structure built without the required permission. Beyond the legal limits, practical shed sizing is worth planning around actual intended use rather than simply choosing the largest size that fits the available space: a shed intended mainly for garden tool storage needs considerably less space than one intended to double as a workshop, home gym, or home office, and it's generally easier and cheaper to buy a somewhat larger shed from the outset than to extend or replace an undersized one once storage needs grow, which is a common and avoidable regret among first-time shed buyers who priced only for their immediate, current storage needs.

Worked Example: Base Materials for an 8ft × 6ft Shed

An 8ft × 6ft shed footprint converts to roughly 2.44m × 1.83m, giving a base area of 2.44 × 1.83 ≈ 4.46 m². The right base approach depends on budget and ground conditions. Concrete slab option: a typical 100mm-thick slab needs a volume of 4.46 m² × 0.1m = 0.446 m³ of concrete, plus a 50-75mm hardcore sub-base beneath it for drainage and stability on most UK clay or loam soils. Paving slab option: using standard 450mm × 450mm slabs (each covering 0.2025 m²), you'd need roughly 4.46 ÷ 0.2025 ≈ 22 slabs, laid on a compacted sand/hardcore bed, with a slight fall away from the shed for drainage.

Whichever base you choose, it should finish a few millimetres larger than the shed's own footprint so the floor bearers sit fully supported, and level to within about 5mm across the whole base — an unlevel base is the single most common cause of doors sticking and floors flexing in flat-pack sheds.

Roofing Material Options for Garden Sheds

The roof covering affects both lifespan and maintenance far more than most buyers expect. Standard roofing felt is the cheapest option but typically needs replacing or re-coating every 5-10 years as it degrades under UV exposure. Shingle-style roofing (overlapping bitumen or composite shingles) looks closer to a house roof and usually lasts 15-20 years. EPDM rubber roofing, increasingly popular for workshop-grade sheds, is the most durable at 20-30+ years but costs noticeably more per square metre and is more involved to fit correctly. Whichever covering is chosen, the roof pitch matters almost as much as the material — a shallow pitch below about 10 degrees will shed rainwater more slowly and is more prone to felt failure at the seams over time.

What should I know about Shed Base Drainage?

A shed base that doesn't drain properly is one of the most common causes of premature timber rot, even in sheds built from decent-quality wood. A slight fall of around 1:60 (roughly 15-20mm over a 1m width) away from any adjacent wall or fence stops water pooling against the shed's lowest timber rail. For a slab or paving-slab base, a sub-base of 75-100mm of compacted hardcore topped with a sand blinding layer allows water to drain through rather than sitting under the slab, and a shed base should always sit slightly above the surrounding ground level rather than flush with it or recessed.

Timber Treatment and Guarantee Terms

Shed guarantees vary enormously in what they actually cover, and 'guaranteed against rot' claims are only as good as the treatment method and the small print behind them. Dip-treated sheds (timber briefly dipped in preservative) typically carry a shorter guarantee, often 1-2 years, since the treatment only penetrates the surface layer. Pressure-treated (tanalised) timber, where preservative is forced into the wood under vacuum pressure, penetrates far deeper and commonly carries 10-15 year anti-rot guarantees from reputable manufacturers. Almost all such guarantees are conditional on the base being built correctly (off the ground, with drainage) and on the owner reapplying a timber treatment or preservative periodically, typically every 1-2 years — skipping that maintenance is the single most common reason a guarantee claim gets refused, regardless of how good the original treatment was.