How to Price a Balcony Installation: Labour, Materials and Margin Guide

Summary

"Balcony" gets used loosely on site to mean three structurally different things, and pricing them the same way is the single biggest quoting error in this trade: a Juliet balcony (a safety barrier across a door opening with no projecting platform — see juliet balcony installation and juliet balcony installation pricing guide for that job specifically), a standalone balcony (a platform supported on its own posts down to the ground, structurally independent of the house), and a cantilevered balcony (a platform that projects from the building with no ground support, carried entirely by steel or engineered timber tied into the existing floor structure). This guide covers the two platform types — standalone and cantilevered — where a customer can actually walk out onto the balcony. If the job has no platform, price it as a Juliet balcony instead.

The commercial risk with balcony work is underpricing the structural and regulatory overhead. A cantilevered balcony isn't a bigger version of a Juliet balcony job — it's inserting new structural steelwork into an existing floor, waterproofing a deck that sits over habitable space below, and guarding a fall of potentially 2-3 metres rather than a doorway threshold. Get the structural engineering, the waterproofing detail, and the planning route wrong in the quote and the job either loses money on remedial work or fails Building Control.

This article breaks the job into standalone vs cantilevered construction, prices the balustrade and waterproofing separately (because they're priced and supplied differently), and gives worked examples for both structural types. It assumes the reader already understands Juliet balcony pricing from juliet balcony installation pricing guide and is quoting a genuine walkable platform.

Key Facts

• Standalone balcony — supported on its own posts (timber or steel) running to ground-bearing pad foundations or piled footings; structurally independent of the house wall, which typically only takes a lateral restraint fixing, not the full vertical load
• Cantilevered balcony — projects from the building with steel beams (or engineered timber) built into or bolted onto the existing floor joists/frame, carrying the full load back into the house structure with no ground support; always needs a structural engineer's design
• Guarding height — minimum 1100mm from the finished deck level to the top of the balustrade, per Approved Document K, same as Juliet balconies and stairs
• 100mm sphere rule — no gap in the balustrade should allow a 100mm diameter sphere through, to prevent small children slipping through or getting wedged
• Barrier loading — balustrade design load is set by BS 6180 based on the building's occupancy category; get a structural calculation or manufacturer's certified design rather than assuming a figure —
• BS 8579 — the code of practice covering the design and installation of balconies and terraces specifically, including waterproofing, drainage falls and structural interface detailing; increasingly the reference standard specifiers and Building Control expect balcony designs to follow
• Permitted development exclusion — the Town and Country Planning (General Permitted Development) Order 2015 (as amended), Schedule 2 Part 1 Class A, specifically excludes the erection of a veranda, balcony or raised platform from permitted development rights — meaning almost every walkable balcony with a deck raised more than around 300mm needs a full planning application, regardless of size
• Juliet balconies generally escape this — because there's no raised platform to stand on, a Juliet balcony conversion is commonly treated as a standard window/door alteration under permitted development (subject to the usual elevation and conservation area caveats) — this is the practical reason Juliet balconies are so much more common than true platform balconies on standard housing stock
• Waterproofing — a cantilevered or standalone deck over habitable space below needs a proper waterproofing system (single-ply membrane, liquid-applied, or GRP) designed to fall to a drainage outlet, not timber decking laid directly onto structure with gaps and hope
• Thermal bridging — where steel cantilever beams pass through the building's insulation line into an unheated/external zone, they create a cold bridge under Approved Document L; thermal break systems (structural thermal break connectors) are increasingly specified to break the steel's continuity through the insulation
• Fire — combustible materials restriction — balconies attached to relevant buildings over 11m in height are restricted to non-combustible construction (materials achieving European Class A2-s1,d0 or better) under amendments to the Building Regulations fire safety provisions introduced following Grenfell — relevant on taller residential blocks, not typical two-storey house extensions
• Building Notice or Full Plans — cantilevered structural work into an existing floor almost always goes through Building Control as a Full Plans or Building Notice application given the structural design involved; a competent-person self-certification route is not normally available for new structural balcony work
• Party Wall etc. Act 1996 — relevant where balcony fixings, foundations, or scaffolding affect a shared boundary or party structure with a neighbour
• Decking finish — timber, composite, or porcelain paving on pedestal supports are the common finishes over the waterproof membrane; each has different cost, slip resistance, and maintenance implications and should be priced as a separate line from the waterproofing itself
• Drainage outlet — every balcony deck needs a proper outlet and overflow to shed rainwater away from the building fabric — an afterthought outlet is a common source of later leaks and callbacks

Quick Reference Table

Detailed Guidance

Deciding structure type before quoting

The first site visit has to answer one question before any price is written down: does the platform sit on its own posts, or does it hang off the building? A standalone balcony is a simpler quote — it's essentially a raised deck (posts, pad foundations, bearers, joists, decking) with a balustrade on top, and the house wall only takes a modest lateral tie, not the deck's weight. A cantilevered balcony is a structural alteration to the building — the existing floor structure has to be assessed (and very often strengthened) to carry a new cantilever load it wasn't originally designed for, which means a structural engineer is involved from the first sketch, not brought in after the quote is accepted.

Never price a cantilevered balcony without first getting a structural engineer's outline input, even if it's a paid pre-quote visit charged to the customer. Guessing at steel sizes and then finding out mid-job that the existing joists need doubling, or that a load-bearing wall below needs additional support, turns a profitable job into a loss.

Pricing the waterproofing and deck

Waterproofing is priced separately from the structure and separately from the finish decking, because all three are different trades and different failure points. A single-ply or liquid-applied membrane system, correctly detailed with upstands, outlets and falls to BS 8579 guidance, typically runs £180-£280 per m² supplied and fitted by a specialist; GRP (fibreglass) decking is a common alternative at a broadly similar cost band and is popular because it's a seamless, jointless finish that doubles as the wearing surface, removing the need for a separate deck finish. If a separate timber, composite, or paving finish is laid over a membrane on pedestals, price that as an additional line — typically £60-£120 per m² for timber/composite decking boards and fixings.

Pricing the balustrade

Balustrade pricing follows the same tiers as the Juliet balcony market — framed metal (£150-£300/m), framed glass (£220-£400/m), and frameless structural glass (£350-£600/m) — but on a walkable balcony the run is usually longer (three sides rather than one door width) and the fall being guarded is greater, so specifiers more often push toward frameless glass for the unobstructed view. Always include the structural loading certificate to BS 6180 as a priced deliverable, not an assumed freebie from the supplier.

Worked example — cantilevered balcony, GRP deck, framed glass balustrade

A 2.4m x 1.5m cantilevered balcony off a first-floor rear elevation, steel beams tied into the existing floor joists, GRP waterproof deck, framed glass balustrade on three open sides (5.4m run):

This sits comfortably inside the £8,500-£16,000 range for a mid-tier cantilevered balcony — the structural engineering and steelwork are what push this well above a standalone deck, even before the balustrade tier or a frameless glass upgrade is considered.

Common pricing mistakes

• Treating a standalone deck-on-posts and a cantilevered balcony as the same job. They have almost nothing in common structurally, and the price difference reflects that.
• Skipping the structural engineer for a cantilever "because it looks like the joists are fine". Existing floor joists are designed for a floor load, not a cantilevered balcony load transferred back through them — always get an engineer's assessment.
• Forgetting planning permission because "it's only a small balcony". The GPDO exclusion for verandas, balconies and raised platforms applies regardless of size — quote for a planning application on almost every genuine platform balcony, and check with the local authority before assuming permitted development on a Juliet-style conversion instead.
• Pricing waterproofing and decking as one line. A membrane failure under a nice timber deck is an expensive strip-out and re-lay; price and warrant the waterproofing separately so it's clear what's covered.
• No cold-bridging detail on cantilevered steel. Thermal break connectors add cost but avoid condensation and heat-loss problems (and Part L compliance issues) where steel punches through the insulation line.

Frequently Asked Questions

Do I always need planning permission for a balcony?

For a genuine walkable platform balcony, yes in almost all cases — the GPDO specifically excludes verandas, balconies and raised platforms from permitted development, unlike most other single-storey extension work. Always confirm with the local planning authority before quoting, and be clear with the customer that planning permission (with its own timescale and fee) is a separate cost and process from the build itself. Juliet balconies, having no raised platform, are usually a different, more permissive case.

What's the difference in price between standalone and cantilevered?

Roughly double to triple. A standalone balcony is a raised timber or steel-post structure independent of the house, similar in complexity to a raised deck — no structural engineer input into the existing building fabric is usually needed. A cantilevered balcony requires structural assessment and often strengthening of the existing floor, steel fabrication, careful waterproofing at the building interface, and a Building Control structural sign-off, all of which add both direct cost and risk that should be reflected in the price.

Does a balcony need a structural engineer even if it's small?

For any cantilevered design, yes — there is no safe way to size a cantilever beam projecting from an existing floor structure without a proper structural calculation, regardless of how small the platform looks. For a standalone deck on its own posts and foundations, a structural engineer is not always required for a straightforward domestic size, but is still recommended where ground conditions are uncertain or the deck height creates a significant fall.

What margin should I be targeting on balcony work?

20-25% is realistic once the structural engineering, Building Control fees, waterproofing specialist (if subcontracted), and balustrade certification are all priced as separate, correctly-costed line items. Balcony work has more subcontracted specialist input than most carpentry jobs (structural engineer, sometimes a waterproofing specialist, sometimes a steel fabricator) — protect margin by marking up subcontracted elements rather than just labour, and by not absorbing the planning application process into a "free" estimate.

Regulations & Standards

• Approved Document A (Building Regulations) — structural requirements; governs the design of any cantilever or new supporting structure.

• Approved Document K (Building Regulations) — guarding height (1100mm minimum) and gap requirements for balconies and raised platforms.

• Approved Document L (Building Regulations) — thermal performance; relevant to cold-bridging where steelwork penetrates the insulation line.

• Approved Document N (Building Regulations) — glazing safety and critical locations, applicable to any glass balustrade; withdrawn in England in 2013 and its requirements folded into Approved Document K (provisions K4/K5), though Part N remains a standalone document in Wales — check under Part K rather than a separate Part N if working in England.

• BS 6180 — code of practice for barriers in and about buildings; basis for the balustrade's structural load certificate.

• BS 8579 — code of practice for the design of balconies and terraces, covering waterproofing, drainage falls and structural interface detail.

• BS EN 12600 — safety glass classification, applicable to any glazed balustrade.

• The Town and Country Planning (General Permitted Development) Order 2015 (as amended), Schedule 2 Part 1 Class A — excludes verandas, balconies and raised platforms from permitted development rights.

• Party Wall etc. Act 1996 — relevant where fixings, foundations or scaffolding affect a shared boundary.

• Building fire safety provisions for balconies on taller buildings — restrictions on combustible balcony materials for relevant buildings over 11m.

• gov.uk — Approved Document A — structural requirements for altered and new structures.

• gov.uk — Approved Document K — fall protection and guarding heights.

• Planning Portal — Permitted Development for Householders — permitted development rights and exclusions, including balconies.

• BSI — British Standards — BS 6180 and BS 8579 standard details.

• Glass and Glazing Federation — trade body technical guidance for balustrade glass.

• juliet balcony installation — the regulatory case for a Juliet balcony (no platform), a distinct and usually simpler job than a true balcony.

• juliet balcony installation pricing guide — pricing for Juliet balcony conversions specifically.

• part k falls — guarding heights and the 100mm sphere rule in detail.

• part a structure — structural alteration requirements referenced throughout this guide.

• glass balustrade installation — installation detail for glass balustrades, framed and frameless.