Roof Terraces in London: Planning, Construction and Cost

A roof terrace is one of the most desirable additions to a London property — and one of the more complex to deliver. The combination of planning constraints, structural requirements, waterproofing demands, and neighbour relations means that a poorly handled roof terrace project can go wrong in multiple ways simultaneously. Done well, it creates amenity space that is genuinely difficult to find anywhere else in central London.

This guide covers what you need to know before starting a roof terrace project: planning, structure, waterproofing, drainage, specification, and realistic costs.

Planning permission

Whether a roof terrace requires planning permission depends on several factors, but the short answer for most of London is: almost certainly yes.

Permitted development

Converting a flat roof to a terrace is not permitted development for most London properties. Unlike some other loft or extension works, creating a new roof terrace requires planning permission in the following situations (which cover almost all central London cases):

• —The property is in a conservation area
• —The property is listed
• —The property is a flat (flats have significantly more restricted permitted development rights than houses)
• —The proposed works exceed the permitted development parameters — which the creation of a new terrace almost always does

For houses outside conservation areas, there is a narrow set of circumstances where a simple roof conversion to a terrace might fall within permitted development, but these are the exception rather than the rule. The safe starting assumption is that planning permission is required.

What planning considers

The planning authority assesses roof terrace applications primarily on the basis of:

Overlooking and loss of privacy. Roof terraces create new elevated vantage points that may overlook neighbouring properties, gardens, and windows. This is the most common ground for objection and refusal. The design of the terrace — the height and position of screens and balustrades, the direction of views, the proposed use — all factor into the assessment.

Character and appearance of the conservation area or building. In conservation areas, new roof structures, balustrades, and any associated plant or mechanical equipment must be designed to be unobtrusive. Glass or frameless systems are generally preferred over opaque timber or masonry balustrades.

Noise and disturbance. A new terrace that would create noise nuisance for neighbours — particularly in tightly-packed terraces where there is little separation between buildings — will face scrutiny.

Loss of daylight or sunlight to neighbouring properties. Less commonly an issue for roof terraces than for extensions, but relevant where a raised structure or screening significantly reduces light to adjoining properties.

Pre-application advice with the relevant borough is strongly recommended before committing to a design. The cost is modest relative to the project, and the outcome — an informal steer from planning officers on what they will and will not support — is invaluable.

Structure

Converting a flat roof to a habitable terrace requires structural assessment. The key questions are:

Can the existing roof structure carry the loads? A terrace adds significant additional loading to the roof: people, furniture, planted containers, paving, waterproofing build-up, and drainage elements. The existing joists may not be sized for these loads, and upgrading them often requires opening up the ceiling below — an intrusive operation that needs to be planned for.

What is the structural specification for the balustrade? Balustrades on terraces are subject to BS 6180 (the standard for barriers in and about buildings), which specifies the horizontal load a barrier must resist. The fixings into the structure are critical — an improperly fixed balustrade that fails under load is a life-safety issue.

Are there any party wall implications? If the terrace structure requires any work to or near the party wall — for example, fixing a balustrade or screen to a shared structure — the Party Wall etc. Act 1996 may be triggered. Party wall notices may need to be served. See our Party Wall Act guide.

A structural engineer's involvement from the design stage is essential for any roof terrace project.

Waterproofing

Waterproofing is the single most critical technical element of a roof terrace — and the one most often inadequately specified. A flat roof that leaks is bad; a flat roof that leaks after it has been covered with paving, drainage layers, and furniture becomes a very expensive problem.

The options for warm flat roof waterproofing (the typical approach in new and refurbished work) include:

Single-ply membranes (TPO, PVC, or EPDM) are the standard for new flat roof work in residential projects. They are lightweight, durable, and — if correctly detailed at junctions, upstands, and penetrations — perform well over a long service life. The detailing at edges, drainage points, and any penetrations (outlets, upstands, fixings) is where failures occur. This is specialist work that requires an experienced contractor and third-party inspection.

Hot melt (liquid-applied) systems are more tolerant of complex geometries and penetrations and produce a seamless, fully bonded waterproofing layer. They are more expensive than single-ply but appropriate for complex roofs and projects where the consequences of a leak are particularly severe (the roof above a principal bedroom, for instance).

GRP (glass-reinforced plastic) is commonly used for smaller balconies and terraces. It is strong, seamless, and easily repaired, but can be noisy in rain and has limited suitability for large areas.

The waterproofing layer should always be specified separately from the finished surface — the paving or decking is a wear layer on top of the waterproofing, not a substitute for it.

Drainage

Drainage design is as important as waterproofing. A roof terrace must drain efficiently — water that ponds on the surface or backs up behind balustrades creates loading, accelerates wear, and may penetrate inadequate details over time.

Design for a minimum fall of 1:80 to all outlets. Multiple drainage points for any terrace over 10m² are standard practice. Outlet locations need to be coordinated with the waterproofing design and the finished surface layout before the structure is finalised.

Specification decisions

Paving. The floor surface needs to be suitable for external use, non-slip when wet, and compatible with the waterproofing system. Porcelain and natural stone on pedestal systems (which allow drainage beneath and avoid mechanical fixings through the waterproofing) are the standard approach for high-specification terraces. Timber decking is an alternative for a warmer aesthetic but has ongoing maintenance requirements.

Balustrade and screening. The design of the balustrade affects the visual appearance of the terrace from the street, the privacy it provides, and the planning acceptability of the scheme. Frameless glass is widely used in conservation area applications — it is visually lightweight and preferred by planning authorities. Perforated metal screens, cable systems, and structural glass fins are alternatives depending on the design context.

Planted elements. Planters and green elements add significantly to the weight loading and require their own drainage. They need to be designed as part of the structural scheme, not added afterward.

Rooftop plant and services. If air handling units, heat pump condensers, or other mechanical equipment is to be located on the roof, this needs to be coordinated at the design stage — in terms of planning (plant on roofs is controlled in conservation areas), structure, and acoustic screening.

Costs

Indicative costs for a roof terrace conversion in London at high specification:

A complete roof terrace conversion for a 30–40m² terrace at full specification typically runs to £60,000–£120,000 including planning, structure, waterproofing, paving, and balustrades. Larger or more complex projects, or those in sensitive conservation area locations, will be at the higher end.

If you would like to discuss a roof terrace project, contact us to arrange a site visit. You can also see examples of our renovation and extension work in our portfolio.