Roof Terraces in London 2025: Planning, Design, and Cost Guide

**The Permitted Development position on roof terraces**:

Roof terraces — by which we mean an accessible flat-roof deck used as an outdoor amenity space — are explicitly excluded from Permitted Development under the General Permitted Development Order (GPDO) in England. Specifically:

• •Class A of Part 1 of Schedule 2 (Householder PD) permits certain extensions and roof alterations, but the enlargement, improvement, or alteration of a dwelling in a way that creates or materially extends a roof terrace is specifically excluded
• •**Any roof terrace on the main roof of the house requires planning permission** — it is not Permitted Development
• •A flat roof extension that is used as a terrace (walking on the flat roof of the extension and using it as outdoor space) is also not Permitted Development — the GPDO excludes the creation of a roof terrace even on an extension that is otherwise PD

• This means: **if you want a roof terrace in London, you need full planning permission**, regardless of:
• •Whether the roof is on an extension or the original house
• •Whether the terrace is accessible from a door or window that already exists
• •Whether you are in a conservation area or not

The planning permission requirement applies to the access and use as amenity space — even if the flat roof structure itself was permitted as part of an extension (or was already existing), adding a balustrade, decking, and using the roof as outdoor space constitutes a material change requiring planning consent.

**Why London planning authorities typically refuse roof terrace applications**:

The planning consideration that drives roof terrace refusals in London is **overlooking impact** — the loss of privacy to neighbouring properties. London Victorian terraces and their extensions are densely packed: a flat-roof extension terrace at first-floor level (approximately 4–6m above ground) typically gives direct views into the rear windows and gardens of all neighbouring properties within a 10–20 metre radius. In densely-developed areas, this is considered an unacceptable loss of privacy to neighbours under:

• •**NPPF (National Planning Policy Framework) paragraph 135(f)**: a material consideration requiring that development should 'not result in an unreasonable deterioration in the daylight or privacy experienced by occupants of existing buildings'
• •**London Plan Policy D6**: design standards requiring that new residential development avoids unacceptable overlooking impacts
• •**Local planning policies**: most London boroughs have specific residential design guidelines that define maximum acceptable separation distances between windows/terraces and neighbouring garden/living-space (commonly 12m to 25m from a terrace to a neighbouring window — distances typically not achievable from a rear-extension roof terrace in a London Victorian terrace street)

*The specific refusal scenario*:

A typical London terrace rear extension has a flat roof at approximately 2.5m–3.5m above ground level (first-floor extension height). A roof terrace at this height gives direct over-the-fence views into all neighbouring rear gardens and through rear bedroom/bathroom windows within a line-of-sight radius of typically 20–40 metres. In a densely developed inner London residential street, every neighbour within that radius can claim overlooking impact — and planning officers, under pressure to refuse roof terraces, typically recommend refusal with 'overlooking' as the reason.

**When do roof terrace applications succeed in London?**:

Successful roof terrace applications in London typically have one or more of the following characteristics:

1. **The property backs onto a non-residential use**: where the rear of the property faces onto a commercial area; a park; a road; or other non-residential uses, there are no neighbouring residential rear windows/gardens to be overlooked — the overlooking objection evaporates

2. **The terrace is set back significantly from the rear boundary**: on a deeper rear extension or a larger plot, a terrace with a substantial setback from the rear boundary (5m or more) and with 1.7m minimum-height privacy screening on the side boundaries can demonstrate that neighbouring rear gardens are not materially overlooked. This argument is easier to make with a plan view showing the sight-lines

3. **High-level screening** that demonstrably blocks views into neighbouring properties: a roof terrace with a solid parapet or glazed balustrade system at 1.8m above terrace level (effectively screening the neighbours) can be argued as non-overlooking. However, planning officers are sceptical of screening arguments because: (a) screening can be removed after consent is granted; (b) the screening itself may be visually intrusive; (c) even with screening, a terrace at height creates noise impact from social use

4. **The roof is at upper-floor level (second or third storey)**: at higher levels, the view is more likely to be into the sky than into neighbouring windows and gardens — overlooking impact reduces as height increases, and at the second or third floor level a roof terrace may look out over neighbouring roof lines rather than into them. High-value London properties with sufficient height to look over the surrounding building stock are more likely to achieve roof terrace planning consent

5. **Planning history in the area**: where several neighbouring properties already have roof terraces (and therefore the character of the area already includes this form of development), a new application is harder to refuse on character grounds — though overlooking of specific neighbours remains a material consideration

**Party Wall implications of a roof terrace**:

Accessing a flat-roof extension terrace typically requires a door or hatch in the rear wall of the main house at first-floor level. Creating this opening in the party wall (if the rear wall is a party structure) requires a Section 2 Party Wall Notice. The terrace use itself does not directly trigger the Party Wall Act, but the overlooking impact may cause neighbours to respond to the planning application with objections — and may prompt them to assert that the terrace harms their amenity under other grounds.

**Structural considerations — can the flat roof support a roof terrace?**:

A standard flat-roof extension is designed with the minimum structural load capacity required for an inaccessible roof subject to maintenance access and snow load. A roof terrace has significantly higher imposed loads:

• •Inaccessible flat roof (maintenance access only): imposed load typically **0.75 kN/m²** in structural design
• •Accessible roof terrace (publicly accessible or private amenity): minimum imposed load **1.5 kN/m²** (for a residential roof terrace that will not be used for assembly/gathering — for a terrace with a table and chairs; occasional social use)
• •Roof terrace with planting/containers: add **0.5–1.5 kN/m²** per container depending on weight of growing medium; large planters may require specific point-load analysis
• •Roof terrace with a hot tub: hot tubs fully loaded may weigh 2,500–4,000 kg — this requires specific structural engineering analysis and the joist/beam sizing to support it

The structural engineering assessment for a roof terrace must confirm: 1. Whether the existing joists/rafters supporting the flat roof deck are capable of the additional imposed load 2. Whether the waterproofing system is capable of being walked on (not all flat roof waterproofing membranes are suitable for regular pedestrian traffic) 3. Whether the joist span is adequate or whether mid-span support (additional beams from below) is required

For most London rear extensions, the flat roof is typically built on 200mm joists at 400mm centres (or 175mm joists) spanning the extension width (typically 3.0–5.5m). The structural engineer's assessment will determine whether these can carry a 1.5 kN/m² imposed load or whether upsizing is required. In a new-build extension intended to include a roof terrace from design stage, joists can be upsized during construction at marginal additional cost. In a conversion of an existing flat-roof extension to a terrace (after the original build), a structural engineer's report and possible joist strengthening works are required.

**Waterproofing for a walkable roof terrace**:

Not all waterproofing membranes are suitable for pedestrian use without protection:

• •**GRP fibreglass**: not suitable for regular pedestrian use without a sacrificial non-slip coating or decking system over the top. GRP will scratch and eventually crack under repeated foot traffic; temperature cycling causes GRP to expand and contract and this movement under foot traffic stresses the laminate at fixings and edges
• •**EPDM rubber membrane**: marginally more durable for pedestrian use but still not ideal for direct foot traffic — typically requires a decking system over the top (composite decking; timber decking) supported on pedestals above the membrane to protect it
• •**TPO/PVC single-ply**: hot-air welded single-ply membranes are more resistant to pedestrian traffic than GRP but still benefit from a decking system above for long-term protection
• •**Liquid-applied PMMA (Kemperol; Triflex)**: the most durable option for a walkable surface — a liquid-applied PMMA system with a non-slip aggregate broadcast into the top coat provides a seamless, durable, directly-walkable surface that is resistant to UV, ponding, and regular foot traffic. A PMMA surface can be specified to provide a directly-walkable finish (RAL colour with aggregate) without the need for a decking system above, which simplifies the terrace design and eliminates the risk of water being trapped under decking
• •**Composite decking on pedestals over membrane**: the most common London roof terrace system — any of the above waterproofing membranes, with composite or hardwood timber decking boards laid on adjustable-height pedestals (typically aluminium or polypropylene pedestals at 150–400mm height adjustment, allowing the deck to be levelled over any fall in the roof surface). The gap between the decking board and the membrane allows water to drain freely to the outlets under the deck. Advantage: the decking conceals and protects the membrane; the surface is attractive. Disadvantage: water ponding under the deck can be difficult to inspect; outlet blockage under the deck can result in significant water build-up before detection

**Balustrade and guarding — Building Regulations Part K**:

Any accessible roof terrace where the terrace surface is more than 600mm above adjacent ground level must have guarding (balustrade) that complies with Part K of Building Regulations. Requirements:

• •Minimum height of guarding from the finished deck surface to the top of the balustrade: **1,100mm** for any accessible roof terrace in a domestic building (not the lower 900mm minimum that applies to some internal stairs)
• •The balustrade must be designed to resist a horizontal loading of 0.74 kN/m (residential)
• •No climbable horizontal rails or features below 1,100mm that a child could use to climb over the balustrade — all horizontal members must be above 760mm height or designed with vertical rails at ≤100mm spacing
• •The structural frame of the balustrade must be fixed to the structural floor of the terrace (not to the waterproofing membrane surface — through-membrane post fixings require specialist sealing to maintain waterproofing integrity; sleeved post systems are preferred where the post sleeve is cast into the concrete edge or fixed to the structural joist)

• *Balustrade options for London roof terraces*:
• •**Structural glass (point-fixed; glass fin; frameless)**: most desirable in terms of maintaining views and maintaining sense of space; glass panels 15–21.5mm toughened laminated safety glass; IQ Glass; Vision Glass; Balconette. Premium cost but most attractive and least visually intrusive from neighbouring gardens. Supply: £350–£600 per linear metre of frameless structural glass balustrade; installation: £200–£400 per linear metre
• •**Aluminium and glass (framed)**: powder-coated aluminium post and rail system with glass infill panels (Balconette; Rintal; Origin). More economical than frameless glass; durable; minimal maintenance. Supply and install: £250–£450 per linear metre
• •**Powder-coated steel post and cable**: stainless steel cable strung horizontally between posts — transparent appearance but cables must be tensioned correctly (sagging cables in cables railings are both non-compliant and aesthetically poor). Each cable spacing must be ≤100mm. Supply and install: £200–£350 per linear metre
• •**Timber balustrade**: treated hardwood; visually warm; requires maintenance (oiling/treating every 2–3 years). Supply and install: £150–£300 per linear metre

**Access to the roof terrace — door specification**:

• A roof terrace accessed from the first floor of the main house requires a door (bifold; French door; or single-opening door) in the rear elevation of the house at first-floor level. This door:
• •Must be thermally compliant (max U-value 1.6 W/m²K for the glazed unit)
• •Must have a low-threshold or flush threshold to allow easy access from the first-floor room level to the terrace (the decking surface must be designed at or below the internal floor level to avoid a trip hazard at the threshold)
• •The structural opening in the rear wall to accommodate the door may require a steel or concrete lintel — if the rear wall is the party wall (the side wall of the terraced house) this requires a Party Wall Notice

**Cost summary for a typical London rear-extension roof terrace conversion (2025)**:

| Element | Cost range | |---|---| | Structural engineer's assessment + design for terrace loading | £800–£2,000 | | Joist strengthening (if required; labour and materials) | £1,500–£4,000 | | PMMA liquid-applied walkable waterproofing on existing flat roof (20–25m²) | £4,500–£7,500 | | Composite decking on pedestals over existing membrane (20–25m²) | £3,500–£6,000 | | Structural glass balustrade (per 8 linear metres perimeter) | £4,000–£8,000 | | Aluminium and glass balustrade (per 8 linear metres) | £2,500–£5,000 | | External door / bifold to access terrace | £2,500–£6,000 | | Planning application fee and architect / planning consultant | £2,000–£5,000 | | **Total for a 20–25m² terrace conversion (structural glass; planning; full design)** | **£15,000–£35,000** |

**When a roof terrace application is refused — alternatives that achieve similar outcomes**:

Given the high rate of refusal for roof terraces in dense London residential areas, it is worth understanding the alternatives before committing to a planning application that has a low probability of success.

*1. Juliet balcony on the rear elevation*:

A Juliet balcony (a French door or large opening window with a fixed balustrade across the external opening at floor level) does NOT create a terrace — there is no platform to stand on; the balustrade is fixed to the opening surround and allows the door to open while providing a safety guard. Juliet balconies on the rear elevation of a house are typically Permitted Development (subject to the standard PD limits on glazed area and projection) and do not create overlooking impact because there is no standing area from which to look. A Juliet balcony provides the feel of connection to outside space and the view from a bedroom or first-floor room without the overlooking issue.

*2. Terrace at second-floor level (loft conversion)*:

On a three-storey house with a loft conversion, a rear-facing terrace cut into the mansard roof slope (an 'inset terrace') at second-floor level — where the terrace is recessed into the building rather than a flat-roof terrace above the extension — may have a better prospect of planning consent because: (a) the surrounding building stock at second-floor height may be at a lower level (looking over the rooftops of single-storey extensions and two-storey rear additions rather than directly into ground-floor gardens); (b) the angled sight-lines at height are less direct into neighbouring windows. This is not guaranteed — many London boroughs refuse inset terraces in loft conversions on overlooking grounds — but the prospect is better than a first-floor extension terrace.

*3. Enclosed courtyard garden maximisation*:

In London Victorian terraces where the rear garden is short (5–10 metres), investing in high-quality garden design (raised planting; outdoor kitchen; pergola structure; hard landscape) can create a more usable outdoor space than a roof terrace that may receive planning refusal. A quality courtyard garden design for a 5m × 8m rear garden in London: £15,000–£40,000 for a full hard-landscape design and build — comparable to or less than the cost of a roof terrace including planning, structural work, waterproofing, and balustrade.

*4. Green roof (non-accessible) as a planning-acceptable alternative*:

Where a planning application for a roof terrace is refused, the refusal is almost always solely on overlooking grounds — not on grounds of structural impact or appearance. The planning officer typically suggests that a green roof (non-accessible sedum or biodiverse roof) would be acceptable. A green roof delivers environmental benefits (London Plan Policy G5); provides insulation; and is often supported by the local planning authority as the preferred roof finish for visible flat-roof extensions. If your planning application was refused and the officer suggested a green roof, a biodiversity net gain argument incorporating a green roof can sometimes unlock approval for an extension that was otherwise borderline on appearance grounds — while accepting that the terrace use is not achievable.