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

Quick Answer: Decking over 300mm above ground level requires planning permission. Decking over 600mm high requires Building Regulations Part A notification for structural adequacy, and Part K guarding — 1,100mm minimum height with maximum 100mm baluster gaps. Timber decking boards should be minimum 25mm thick on joists at maximum 400mm centres. The Permitted Development reference is Schedule 2, Part 1, Class E of the GPDO 2015.

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

Quick Reference Table

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Deck Height Planning Permission Building Regs (Part A) Building Regs (Part K Guarding)
Up to 300mm Not required (PD) Not required Not required
300–600mm Required (exceeds PD) Consult BCO Not required
Over 600mm Required (exceeds PD) Required (structural) Required (1,100mm guarding)
Any height, >50% curtilage Required Depends on height Depends on height
Any height, in designated area Consult LPA Depends on height Depends on height

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:

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:

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:

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:

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