Decking Planning Rules and Building Regs: Height Thresholds, Guarding and Part A Notifications

Summary

Garden decking is subject to the same Permitted Development framework as other outbuildings — it falls under Schedule 2, Class E of the GPDO 2015. The 300mm height threshold is the key planning trigger: decking at or below 300mm above ground level is generally permitted development; decking above 300mm requires a planning application. This threshold exists because elevated decking can significantly affect the privacy of neighbours, overlooking first-floor rooms and gardens in a way that low-level decking does not.

Building Regulations come into play differently from planning. Any decking over 600mm above the adjacent ground level creates a fall risk that triggers the guarding requirements of Approved Document K. Structural adequacy under Part A is also relevant for elevated or large deck structures. For decking close to the house, drainage implications, damp-proof course protection, and the relationship to the existing building structure also need careful consideration.

Understanding the interaction between planning rules and Building Regulations — and the separate question of the Permitted Development curtilage limit — allows builders and landscapers to advise clients correctly and price work that will be legally compliant.

Key Facts

• 300mm height threshold (planning) — decking over 300mm above ground level requires planning permission in most circumstances; measured at the top surface of the deck board
• 50% curtilage rule — decking counts toward the 50% coverage limit under Class E PD; combined with all other outbuildings, must not exceed half the original dwelling curtilage
• 20m from highway — decking within 20m of the boundary of a highway (including public path) is subject to additional scrutiny; consult LPA
• Designated areas — in Conservation Areas, AONBs, National Parks, the 300mm threshold is lower; any deck visible from a public area may require consent
• Part A structural — raised decks (over 600mm) require Building Control consideration; a Building Notice or Full Plans application should be submitted
• Part K guarding — any deck edge over 600mm above adjacent ground requires guarding; residential: 1,100mm minimum height
• 100mm baluster gap — maximum gap between balusters; this prevents a 100mm sphere (child's head gauge) from passing through
• Post sizes — 100×100mm minimum for deck posts up to 3m height; 125×125mm for posts over 3m or carrying a pergola
• Joist size — 50×100mm joists at 400mm maximum centres as a standard residential starting point; check span tables for actual loads and spans
• 25mm deck boards — minimum 25mm thick boards (not 22mm or 19mm); thicker boards deflect less and resist splitting better
• 3mm drainage gap — minimum gap between deck boards for drainage; 5mm recommended for wet climates; wider gaps allow debris to fall through but reduce safety for high-heeled shoes
• C24 or treated softwood — structural decking components (joists, posts, beams) must be C16 minimum, C24 preferred; treat to UC3 for above-ground external use, UC4 for ground contact
• BS 8417 — preservation treatment of timber; use class selection for decking applications
• Building Notice route — for most residential decking Building Regulations projects; simpler than Full Plans but requires BCO inspection at key stages
• Sloping sites — decks on steeply sloping sites can exceed 600mm at one end even with a level deck surface; check at all four corners

Quick Reference Table

Detailed Guidance

Permitted Development Rules for Decking

Decking is treated as an outbuilding under Schedule 2, Part 1, Class E of the GPDO 2015. The same rules that apply to garden rooms apply to decks:

• Must be within the curtilage of the dwelling
• Combined coverage of all outbuildings and decking must not exceed 50% of the curtilage
• Must not be between the principal elevation of the house and a highway
• In designated areas (Conservation Area, AONB, National Park, World Heritage Site), any structure visible from a public road or footpath requires consent

The 300mm height threshold is specific to decking. Below 300mm, the deck is low-level and raises no overlooking concern. Above 300mm, the deck platform is raised enough to create overlooking potential and potential impact on the amenity of neighbours.

Measuring the 300mm: the height is measured from the existing natural ground level (before any excavation) to the top of the deck board surface. On a level site this is straightforward. On a sloping site, the measurement should be taken at the point where the ground level is lowest relative to the deck surface — typically at the lowest corner of the deck.

What constitutes a highway: for the 20m exclusion zone, highway includes all public roads, public footpaths, and bridleways. It does not include private drives. If the rear garden abuts a public footpath, decking within 20m may require specific LPA consideration.

Building Regulations: Part A Structure

For residential decking projects, Building Regulations Part A (Structure) is relevant where:

1. The deck is raised more than 600mm above ground level at any point
2. The deck carries significant loading beyond normal domestic use (hot tub, water feature, planters with soil)
3. The deck is attached to the main structure in a way that could affect the building's structural integrity
4. The deck involves excavation for footings that could affect adjacent foundations

A Building Notice application is the standard route for most residential decking projects. This notifies Building Control of the intent to carry out work; a BCO inspects at key stages (foundation excavation and concrete, timber structure, guarding installation). No detailed plans need to be submitted in advance, but the work must comply with the Building Regulations when inspected.

A Full Plans application provides more certainty — plans are submitted and approved in advance, and if the BCO approves the plans, the builder is protected against the risk of a prohibition notice on the day of inspection. This is advisable for complex or large-scale decking projects.

Part K Guarding Requirements

Approved Document K (Protection from falling, collision and impact) requires guarding wherever a deck edge is more than 600mm above the adjacent area. For residential buildings, guarding must be:

• Minimum height: 1,100mm measured from the deck surface to the top of the guarding
• Maximum baluster gap: 100mm — no opening that allows a 100mm sphere to pass through; this is the child protection criterion
• Structural: 0.74 kN/m — the guarding must resist a horizontal load of 740N per metre of length at the top rail; this is approximately the force of a person leaning against the rail

For very large decks accessible to the public (commercial premises, restaurants, licensed premises), the loading requirement is higher — consult a structural engineer.

Common guarding options:

1. Timber post and rail — structural posts (100×100mm) at maximum 1.8m centres, with a top rail, a mid-rail, and vertical infill balusters at maximum 100mm centres. The posts must be securely bolted to the joist structure or to separate post anchor bases.

2. Steel or aluminium balustrade — prefabricated systems (Trex Transcend, Deceuninck, TimberTech) with engineering calculations provided by the manufacturer. The anchor bolts must be designed to transfer the 0.74kN/m load to the deck structure.

3. Glass infill panels — toughened or laminated safety glass panels in a post-and-channel system. Requires structural design by the balustrade manufacturer. More expensive but offers better views.

Horizontal rail exclusion: horizontal rails create a climbing frame effect for children. Where children are likely to use the deck, specify vertical infill balusters only — no horizontal intermediate rails between the top and bottom of the guarding.

Timber Specification and Treatment

Deck boards: minimum 25mm thick boards; 28mm or 32mm preferred for spans over 400mm between joists. Standard widths are 95mm, 120mm, and 145mm. Wider boards are more prone to cupping (bowing across the width) as they dry — specify pre-dried boards or use a narrower width.

Drainage gap: 3–5mm gap between boards. Install boards with 3mm spacers and nail or screw at each joist crossing. Use stainless steel fixings — galvanised fixings can rust and stain the decking surface within 2–3 seasons.

Joist specification: 50×100mm (2"×4") at 400mm centres is the standard for residential decking carrying domestic loads. For spans over 2.4m, increase to 50×150mm. For spans over 3.6m, a mid-span beam and additional posts are needed to keep joist spans within limits.

Post and beam: 100×100mm posts at maximum 1.8m centres for decks up to 600mm high. For higher decks, increase to 150×150mm. Posts must be treated to UC4 if set in concrete or in contact with the ground, or UC3 if supported on post bases above ground.

Treatment to BS 8417: all external structural timber should be treated to appropriate Use Class:

• UC3b — above ground, fully exposed (joists, beams, deck boards — apply annual oil)
• UC4 — ground contact (posts set in concrete, sleeper timbers)

Preferred UK timber species for decking: Siberian larch (durable, attractive grain, natural Class 3 durability), Western red cedar (light, stable, Class 2–3 durability), pressure-treated Southern yellow pine, or Accoya (acetylated softwood, Class 1 durability rating).

Drainage, DPC and Structural Connections to the Building

DPC protection: when a deck is attached to a masonry wall, the deck surface must be at least 150mm below the damp-proof course of the building. Water trapped between a deck and a masonry wall can bridge the DPC and cause damp penetration into the house.

Drainage clearance: the space between the underside of the deck joists and the finished ground level should be at least 150mm for air circulation, and ideally 300mm for inspection and maintenance access. Compact a gravel layer (50mm angular gravel) under the deck to suppress weed growth and improve drainage.

Ledger board connections: where a joist spans from a ledger board bolted to the house wall to a freestanding post, the ledger connection must be waterproofed — use joist hanger hardware (Simpson Strong-Tie, MiTek) with a rubber or metal flashing behind the ledger board to prevent water ingress at the wall connection.

Sloping Sites and Retaining Requirements

On steeply sloping sites, a deck that appears low from the house can be more than 600mm above grade at the downhill end — triggering both the guarding requirement and a potential need for a retaining structure beneath the deck.

Where the ground drops away under the deck:

• Calculate the deck height above natural ground at all four corners
• If any corner exceeds 600mm, guarding is required on all exposed edges
• If the slope is retained by the deck structure itself (posts bearing against a cut slope), a retaining wall calculation is needed
• For retaining heights over 600mm, a structural engineer should check the retaining structure

Frequently Asked Questions

Can I build a deck over 300mm high without planning permission?

No — decking over 300mm above natural ground level exceeds the Permitted Development limits under Schedule 2, Class E. A planning application is required. Contact your Local Planning Authority — in many cases, approval is straightforward if overlooking impact on neighbours is minimal.

Does a Juliet balcony on a house count as decking under Part K?

No — a Juliet balcony is part of the building's external wall structure and is governed by Part K in the same way as internal guarding. The requirements are similar (1,100mm for residential, 1m for common areas in apartments) but the regulatory route is Approved Document K, Part B (fire escape), and Part A (structural), not the PD decking thresholds.

My deck is at the same level as the house threshold — do I need guarding?

Only if the deck edge is more than 600mm above the adjacent ground level. For a level site where the threshold is, say, 450mm above the patio level below, guarding is not technically required by Building Regulations. However, it is good practice to provide at least a step guard rail for safety, particularly if the deck edge is unfenced and adjacent to a path or driveway.

What oil should I use on softwood decking?

Use a penetrating decking oil or stain (not a surface paint) specifically formulated for exterior softwood — Osmo UV Protection Oil, Sikkens Cetol HLSe, or Ronseal Ultimate Decking Stain. Apply once per year after cleaning, or as needed when the surface starts to grey and lose water repellency. Test a small area first as different wood species absorb oil differently.

Do I need Building Regs for a deck under 600mm high?

Not typically — a deck under 600mm high and under 300mm above ground level (therefore also not requiring planning permission) falls outside the normal Building Regulations triggers. However, if the deck is attached to the main building in a way that could affect the structure (removing a wall, altering load paths), or if there are drainage implications, Building Control notification may be prudent. Always confirm with your local BCO.

Regulations & Standards

• Town and Country Planning (GPDO) 2015 (Schedule 2, Part 1, Class E) — Permitted Development rights for decking and outbuildings

• Building Act 1984 and Building Regulations 2010 — framework for Part A and Part K requirements

• Approved Document A: Structure (2004, amended 2013) — structural adequacy for raised decks

• Approved Document K: Protection from falling (2013) — guarding requirements; 1,100mm residential height

• BS 8417:2011 — preservation of wood; use class selection for deck components

• BS EN 350 — natural durability of solid wood; species durability classification

• Planning Portal: Decking PD Rules — official summary of PD rules for decking

• Approved Document K — guarding heights and baluster gap requirements

• TRADA: Timber Decking Guide — timber specification for outdoor decking

• Wood Protection Association — preservative treatment use classes (UC3, UC4)

• LABC: Building Control Guidance — Building Notice and Full Plans application process

• garden rooms — PD rules for outbuildings; Class E comparison

• garden steps — step construction and guarding for level changes

• part a structure — when structural engineers are needed