Solar Panels on Extensions London 2026: Planning, Costs & Savings
A new flat-roof extension is one of the best locations for solar panels in London — south-facing, unobstructed, and already requiring a structural engineer. Here is what the planning rules actually say, what installation costs in 2026, and what you can realistically expect to save.
Quick Answer
Solar panels on a home extension are permitted development under Part 14 Class A in most cases — no planning application needed — provided panels do not protrude more than 200mm beyond the roof slope and are not on a wall fronting a highway. Conservation areas and listed buildings have additional restrictions. Costs in 2026: £1,400–£1,600 per kWp installed, 0% VAT until March 2027, typical 4kW system £6,000–£8,500 all-in.
£1,400–£1,600
Cost per kWp
£6k–£8.5k
Typical 4kW system
8–12 years
Typical payback
Check your specific property constraints
Free Property CheckWhy a New Extension Is the Best Time to Add Solar
Most London houses have sloped roofs facing east–west — a compromise for solar generation. A rear extension with a flat or low-pitch roof is different. Flat roofs allow panels to be mounted on frames tilted to the optimal 30–35 degree angle, facing directly south, without the orientation constraints of the main house roof.
The timing also matters. Adding solar during the extension build rather than retrofitting later avoids duplicating costs for scaffolding (already required), electrical first-fix work, and roof penetrations. A structural engineer is already involved in the extension design — routing their assessment to cover solar panel loading costs far less than commissioning a separate structural survey post-build.
What makes flat-roof extensions ideal for solar
Planning Rules: Part 14 Class A Permitted Development
Solar panels on a dwellinghouse are covered by Schedule 2, Part 14, Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015. Under Class A, installation is permitted development — no planning application required — provided the following conditions are met.
Part 14 Class A: the key conditions
Panels must not protrude more than 200mm beyond the plane of the roof slope or wall on which they are mounted. On a flat-roof extension this is measured from the flat roof surface itself — ballasted mounting frames tilting panels at 30–35 degrees can easily meet this, and the 200mm limit applies to the lowest point of the panel edge relative to the roof deck.
On flat-roof extensions, panels tilted at 30–35 degrees will project upward — their upper edge must not exceed the height of the highest part of the main house roof (excluding chimney). For a typical single-storey rear extension, this is almost always satisfied since the main roof ridge is above the extension.
The Class A condition requires that the installation is "sited so as to minimise its effect on the external appearance of the building and the amenity of the area." In practice, flat-roof extensions in the rear garden satisfy this condition automatically because the panels are not visible from the street.
Listed building consent is required separately. Class A permitted development does not apply to listed buildings. This applies to the whole curtilage, not just the listed structure itself.
In a conservation area, solar panels cannot be installed on a wall that faces a highway. On the flat roof of a rear extension, this condition does not apply — rear installations in conservation areas are generally permitted development.
Installations on a scheduled monument are not permitted development. Rare in residential contexts but applicable in a small number of London locations.
For most London homes with a standard flat-roof rear extension, the Part 14 Class A conditions are satisfied without any application. The practical exception is listed buildings — if your property is listed or within the curtilage of a listed building, listed building consent is required regardless of where the panels are sited.
Important note on Article 4 directions: Some London boroughs have Article 4 directions that remove permitted development rights, including for solar panels. Check the planning constraints at your address before assuming Part 14 applies — your architect can confirm this as part of the extension design process.
Conservation Areas and Listed Buildings
Conservation areas
Most of inner London sits within or adjacent to a conservation area. For solar panels, the conservation area restriction under Part 14 is specific: panels cannot be installed on a wall that fronts a highway. A flat-roof rear extension does not front a highway — it faces the garden. This means solar panels on a flat-roof rear extension in a conservation area are typically permitted development, exactly as they would be on a non-conservation area property.
Where panels are proposed on a front roof slope visible from the street, or on a side wall that faces a road, the conservation area restriction does apply and planning permission would be needed. The flat-roof rear extension sidesteps this problem entirely for most properties.
Listed buildings
Solar panels on a listed building — or within the curtilage of a listed building — require listed building consent in addition to any planning permission. The starting point for consent applications is demonstrating that the installation is reversible, does not cause structural damage to historic fabric, and minimises visual impact. Consent is more likely to be granted where panels are positioned away from principal elevations, hidden behind parapet walls, or sited on later additions to the building rather than the listed structure itself.
Factors that help listed building consent applications for solar
- Panels sited on a non-historic addition (e.g. a 1960s rear extension) rather than the listed structure itself
- Not visible from any public vantage point or principal elevation
- Fully reversible fixing method with no cutting into historic masonry or roof structure
- Cabling routed internally rather than visibly on external walls
- Heritage officer pre-application discussion completed before formal submission
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Solar Panel Costs in London 2026
Solar panel prices have fallen significantly over the past decade and are now broadly stable. In 2026, installed cost for residential rooftop systems in London runs at £1,400–£1,600 per kWp. 0% VAT applies on solar panel installation and battery storage under the Energy Saving Materials relief, which currently runs until March 2027. All figures below include 0% VAT on solar and battery hardware; electrical work on the wider installation attracts 20% VAT.
| System size | Typical panels | Installed cost | Extension roof area needed |
|---|---|---|---|
| 2 kWp | 5–6 panels | £3,000–£3,800 | ~8–10m² |
| 4 kWp | 9–11 panels | £6,000–£7,200 | ~16–20m² |
| 6 kWp | 14–16 panels | £8,500–£10,200 | ~24–30m² |
| Battery (5–10kWh) | Add-on | £4,500–£7,500 | Wall-mounted |
Flat-roof mounting premium
Flat-roof installations require tilted mounting frames to achieve the 30–35 degree angle for optimal generation. Ballasted frames (weighted rather than fixed to the roof) are preferred where the roof structure allows, avoiding penetrations into the waterproof membrane. The mounting frame premium over a standard pitched-roof installation is typically £500–£1,000 per system.
London installers also tend to charge 10–15% more than the national average due to access constraints, parking costs, and congestion zone charges. Factor this into any national cost guides you reference.
Additional costs to budget for
Planning an extension and want to know how solar fits with your budget? Our AI gives you a combined cost estimate for the extension and solar system.
Get a combined estimate →Building Regulations
Solar panel installations on dwellinghouses are generally exempt from building regulations under Schedule 2, Class 6A of the Building Regulations 2010 — subject to a number of conditions. However, the exemption is for the solar installation itself. The extension structure, electrical works, and any structural alterations still require building regulations approval in the usual way.
Structural loading (Part A)
A typical solar panel weighs 18–22kg. A 4kWp system of 10 panels adds approximately 200–220kg of dead load to the roof, plus wind uplift forces. Ballasted flat-roof mounting frames add further ballast weight — commonly 15–25kg per frame. The extension's structural engineer must confirm the flat roof, its joists, and the wall plate below can carry the combined load. Where the extension structural engineer is already engaged, this is a modest addition to their brief. Retrofitting solar to an existing extension without a prior structural check is risky — get this confirmed before committing to installation.
Electrical installation (Part P)
The electrical installation — inverter, consumer unit connection, cabling, isolation switches — falls under Part P of building regulations (electrical safety in dwellings). This must be carried out by a competent person registered with an approved scheme (NICEIC, NAPIT, or equivalent) who can self-certify the work. Your solar installer should hold this certification. A completion certificate must be issued and a copy kept with the property records. Do not use an installer who cannot provide a Part P completion certificate.
Waterproof membrane penetrations
Where cabling must pass through a flat roof, penetrations must be properly sealed to maintain the integrity of the waterproof membrane. Ballasted systems that avoid roof penetrations entirely are preferred for this reason. Where penetrations are unavoidable, use only approved lead or EPDM flashing collars and ensure the flat-roof contractor signs off the penetration method before work proceeds — an unsealed roof penetration will not be covered under your flat roof warranty.
MCS certification
To receive Smart Export Guarantee payments from your energy supplier, your solar installation must be certified under the Microgeneration Certification Scheme (MCS). MCS certification confirms both the product and the installer meet defined quality standards. Only use MCS-certified installers — this is a non-negotiable requirement if SEG payments are part of your financial case for solar.
Smart Export Guarantee: Getting Paid for Surplus Electricity
The Smart Export Guarantee (SEG) replaced the Feed-in Tariff scheme in 2020. Under SEG, energy suppliers with 150,000 or more domestic customers are legally required to offer a tariff paying homeowners for electricity exported to the grid from MCS-certified renewable installations. SEG rates are set by individual suppliers rather than government, and vary significantly.
SEG rates in 2026: what to expect
Several suppliers offer fixed-rate SEG tariffs of 15–25p per kWh, typically on 12-month fixed terms. These provide certainty but require you to shop around — the best rates are not necessarily with your import supplier.
Variable tariffs linked to half-hourly wholesale prices can reach 20–25p/kWh at peak times but may pay 4–6p/kWh at off-peak periods. These require a compatible smart meter and suit households with battery storage that can time exports to peak rates.
MCS-certified installation, a smart meter capable of half-hourly readings, and an active SEG contract with your chosen supplier. You can use a different supplier for SEG export than for electricity import — switching export tariff does not affect your import contract.
Important context: A typical 4kWp system in London generates around 3,400–3,800 kWh per year. If you self-consume 50–60% of that generation (realistic with daytime working from home or a household with children), you export 1,400–1,900 kWh. At 20p/kWh SEG, that is £280–£380/year in export income — useful, but the main financial benefit comes from reducing the electricity you buy at 24–28p/kWh, not from export payments.
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Battery Storage: Does It Stack Up for London Extensions?
A home battery stores surplus solar generation during the day for use in the evening — avoiding the need to export at low SEG rates and then import at high rates after dark. In principle this makes financial sense. In practice, the payback arithmetic is more challenging than for the solar panels themselves.
Battery storage: the financial case in 2026
The value of stored energy depends on the spread between the SEG export rate and your import tariff. If you export at 20p and would otherwise import at 26p, each kWh you store and self-consume saves you 6p. To cover a £6,000 battery cost at 6p/kWh benefit requires storing and using 100,000 kWh — roughly 20–25 years at full daily cycling. Battery payback on pure financial grounds is long.
The case improves significantly if you are on a time-of-use tariff where off-peak import rates are 7–10p/kWh and peak rates are 30–35p/kWh. A battery charged on cheap overnight electricity (not solar) and discharged during peak evening hours can generate 20–25p/kWh arbitrage. This use case is independent of solar but benefits from having the infrastructure in place.
A cost-effective compromise: install the inverter, cabling, and consumer unit connection with battery-ready capability during the extension build, but defer the battery purchase until prices fall further or your usage pattern confirms the benefit. The marginal cost of making a solar installation battery-ready at build stage is typically £200–£400 — far less than retrofitting the infrastructure later.
Savings and Payback: Realistic Numbers for London
London is not the sunniest city, but solar performance here is better than the north of England and broadly comparable to much of France. A south-facing flat-roof system at 35 degrees can expect approximately 850–950 kWh of annual generation per kWp of installed capacity — better than an east–west pitched roof split, which typically generates 700–800 kWh/kWp.
| Scenario | System | Annual saving | Payback |
|---|---|---|---|
| Small extension, daytime occupancy | 2kWp, no battery | £350–£450/yr | 8–11 years |
| Typical London home, mixed occupancy | 4kWp, no battery | £600–£800/yr | 8–12 years |
| Larger extension, high electricity use | 6kWp, no battery | £900–£1,100/yr | 9–12 years |
| Typical home with time-of-use tariff | 4kWp + 10kWh battery | £900–£1,200/yr | 12–16 years |
Annual savings figures above combine bill reduction (self-consumed generation at 26p/kWh avoided import cost) and SEG payments (exported generation at approximately 20p/kWh). After the payback period, a solar system on a well-maintained flat roof continues generating for 15–20 more years with minimal operating cost — the panels themselves carry 25-year product warranties from most manufacturers.
London-specific consideration: Higher electricity import prices in London (due to higher average tariffs and usage patterns) actually improve the solar savings case relative to national averages. Each kWh you self-consume avoids paying 24–28p rather than a lower regional rate — increasing the annual saving from the same system output.
The Right Order of Works
To get the maximum cost benefit from combining a flat-roof extension with solar panels, the sequence matters. Decisions made at design stage shape what the solar installation can achieve and how much it costs.
Confirm with your architect that the extension roof will be designed to accommodate solar panels. This affects roof orientation (south or south-southwest preferred), structural specification for panel loading, cable routes from roof to consumer unit, and the position of any rooflights that would reduce the available panel area.
Provide the structural engineer with the indicative system size (kWp) and ballast mounting frame weight. They can confirm compliance with Part A structural requirements as part of the extension structural design — avoiding a separate commission later.
During the electrical first fix on the extension, install conduit from the roof to the proposed inverter position, and from the inverter to the consumer unit. Doing this while walls are open costs £200–£400 in additional materials and labour. Retrofitting these routes later requires chasing out finished plasterwork.
Agree the cable penetration method with the flat-roof contractor before the waterproof membrane is laid. Specify the collar type, sealing method, and warranty implications. If opting for ballasted frames with no penetrations, confirm the fall direction of the roof allows water to drain freely around the frames.
Obtain three quotes from MCS-certified solar installers. Provide them with the structural engineer's confirmation, the conduit routes already in place, and the available roof area. Their quote should be lower than a full retrofit because the preparatory work is already done.
Frequently Asked Questions
Do I need planning permission for solar panels on my extension in London?
In most cases, no. Solar panels on a home extension are permitted development under Schedule 2, Part 14, Class A of the GPDO 2015. The key conditions are: panels must not protrude more than 200mm beyond the roof slope, must not exceed the highest part of the main roof, and must be sited to minimise visual impact. For a flat-roof rear extension, these conditions are almost always met. Exceptions apply for listed buildings (which need listed building consent) and for panels on walls fronting a highway in conservation areas.
Can I add solar panels to a flat-roof extension in a conservation area?
Yes, in most cases. The conservation area restriction under Part 14 only applies to panels on a wall that fronts a highway. The flat roof of a rear extension faces the garden, not the highway — so panels there are typically permitted development even in a conservation area. If an Article 4 direction has removed solar permitted development rights in your area, a planning application would be needed. Check planning constraints at your specific address.
How much do solar panels cost on a home extension in London in 2026?
Installed cost runs at £1,400–£1,600 per kWp with 0% VAT on panels and batteries (VAT relief runs until March 2027). A typical 4kWp system costs £6,000–£7,200 installed. Add £500–£1,000 for tilted mounting frames on a flat roof and a London installation premium of approximately 10–15% over national figures. Optional battery storage costs £4,500–£7,500 for a 5–10kWh system.
What is the Smart Export Guarantee and how do I sign up?
The Smart Export Guarantee (SEG) requires energy suppliers with 150,000+ domestic customers to pay you for electricity exported to the grid from an MCS-certified solar installation. Rates in 2026 range from fixed tariffs of 15–25p per kWh to variable smart tariffs reaching higher peaks. To receive SEG payments you need: an MCS-certified installation, a smart meter capable of half-hourly readings, and an active SEG contract with your chosen supplier. You choose your SEG supplier independently of your electricity import supplier.
How long is the payback period for solar panels on a London extension?
For a 4kWp system on a south-facing flat-roof extension in London, typical annual savings are £600–£800 combining self-consumption bill reduction and SEG export payments. At £6,000–£7,200 installed cost, payback runs 8–12 years. After payback, the system continues generating for 15–20 years with 25-year panel warranties. Adding battery storage extends payback to 12–16 years unless you benefit from a significant time-of-use tariff spread.
Do solar panels on an extension need building regulations approval?
The solar installation itself is generally exempt from building regulations under Schedule 2, Class 6A of the Building Regulations 2010. However, the electrical installation (Part P) must be carried out by a competent person registered with an approved scheme, who self-certifies and issues a completion certificate. The extension structural design must also confirm that the roof can carry the additional panel and mounting frame loading under Part A. A Part P completion certificate is required — do not use an installer who cannot provide one.
Summary
A flat-roof rear extension is genuinely one of the best locations for residential solar in London. Planning is straightforward for most properties under Part 14 Class A — the conservation area restriction is narrower than most homeowners assume, and only listed buildings require consent. Costs have stabilised at £1,400–£1,600/kWp with 0% VAT until March 2027.
The financial case is solid for a well-oriented system: £600–£800/year savings on a 4kWp system, 8–12 year payback, and 25 years of generation thereafter. The key is making solar part of the extension design brief from the start — structural engineer, electrical first fix, and roof penetration strategy should all be coordinated during the build, not retrofitted later.
Battery storage adds cost and extends payback on pure bill-saving grounds, but the case improves if you are on a time-of-use tariff. At minimum, make the installation battery-ready during the build for £200–£400, then add the battery when prices fall further or your usage pattern confirms the value.
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