Yes, you can put solar panels on a house extension roof — and for many UK homeowners it turns out to be a better spot than the main roof. But whether it actually makes sense depends on which way the extension faces, what the roof is made of, and whether the structure can take the load. This guide walks through every check worth doing before you call an installer.
Why your extension roof is worth looking at seriously
Single-storey rear extensions are the most common home improvement project in the UK, and the roof that comes with one is often a genuinely useful solar surface. Unlike the main roof — which may face the wrong way, sit under a chimney's shadow, or be cluttered with velux windows — a flat or shallow-pitched extension roof typically offers a clear, unobstructed platform. If it happens to face south, you may be looking at your best solar asset.
That said, the extension roof isn't automatically the right choice. It tends to be smaller than the main roof, which limits how many panels you can fit. And if it's a flat roof, there are a few extra steps — structural checks, mounting frames, tilt angles — that don't apply to a straightforward pitched installation. Let's go through them one by one.
Pitched extension roof vs flat extension roof: which is better for solar?
The type of roof your extension has makes a real practical difference.
Pitched (lean-to or mono-pitch) roofs
A lean-to or mono-pitch extension roof is the easier installation. Panels are fitted on brackets directly to the roof surface, just like on the main house. The key question is orientation: if the slope faces south, south-east, or south-west, you're in good shape. If it faces north, solar panels generally won't earn their keep — it's worth being honest about that. Our guide to whether your roof needs to face south covers the yield differences by direction in plain English.
One advantage of a pitched extension roof over the main roof: it's often at a lower height, making installation simpler and sometimes cheaper.
Flat extension roofs
Most modern single-storey rear extensions have flat roofs, and these work well for solar — with the right setup. Panels can't simply be laid flat; that causes water pooling, dirt build-up, and a significant drop in output. Instead, they're mounted on angled frames.
The optimal tilt for south-facing flat roof solar in the UK is typically around 30 degrees from horizontal, though most domestic installations use a shallower 15–20 degrees to reduce wind loading and keep the array compact. The generation penalty for a 20-degree tilt versus the theoretical optimum is only around 3–5%, so it's a sensible compromise.
Alternatively, an east-west split array — panels facing east on one side, west on the other — works well on a wider flat roof and spreads generation more evenly through the day.
Does the extension roof need to face south for solar panels to work?
South-facing is ideal, but not essential. A south-west or south-east pitch will still produce a very worthwhile yield. The real advantage of a flat roof is that you choose the orientation — you're not locked in by which way the slope happens to face. Panels can be pointed south regardless of the house's orientation, which is one of the genuinely useful things about flat roof installations.
Where solar on an extension roof becomes harder to justify is when it faces north and there's no flat roof option to compensate, or when the roof is heavily shaded by a back-bedroom wall, dormer, or nearby trees. If shade is a concern, it's worth reading our guide on how shade from trees and chimneys affects solar output before going further.
Does the extension roof structure need to be checked?
Yes — and this step matters more for flat roofs than for pitched ones.
A ballasted flat-roof solar array can impose a distributed load of roughly 25–50 kg per square metre, depending on tilt angle and how much ballast is used to anchor the frames. Most modern reinforced concrete flat roof extensions can handle this; older timber-joisted flat roofs sometimes cannot without reinforcement. A structural engineer's assessment is essential before installation on a flat roof — any reputable MCS-certified installer will flag this and arrange it as part of the survey process.
For pitched extension roofs, the loading from standard on-roof brackets is much lower and the structure is generally adequate, but a good installer will still confirm this during the survey visit.
If you're planning an extension and haven't built it yet, this is the best possible time to specify solar from the outset — you can design the structure to carry the load, specify the correct roof angle, and run conduit during the build rather than retrofitting it later.
Do you need planning permission for solar panels on an extension roof?
In most cases, no. Solar panels on an extension roof fall under the same permitted development rules as panels on the main house. Under Part 14 of the General Permitted Development Order (England), roof-mounted solar is permitted development provided panels don't protrude more than 200mm from a pitched roof slope, don't exceed 600mm above a flat roof surface, and don't rise above the highest part of the roof (excluding chimneys). The flat roof limit was relaxed from 200mm to 600mm in December 2023, which opened up more installation options.
There are exceptions worth knowing:
- Listed buildings: Not permitted development — you'll need both planning permission and listed building consent.
- Conservation areas: Panels on a wall or roof that fronts a highway are not permitted development. Rear extension roofs are usually fine, but check with your local planning authority if in doubt.
- Scotland, Wales and Northern Ireland: The rules are similar but not identical — check with your local authority.
If your extension is relatively new, double-check that it was built with the correct permissions itself — lenders and solicitors sometimes flag extension paperwork, and it's easier to resolve before you add panels. Our dedicated guide to solar panel planning permission covers the full picture.
How many panels can you fit on an extension roof?
That depends on the usable area. A typical single-storey rear extension might have a roof area of 12–25 square metres. Once you account for mounting frame spacing (panels need gaps between rows to avoid self-shading) and edge clearances, a flat roof extension might comfortably accommodate 4–8 panels — typically 1.5–3 kWp of capacity.
That's a modest system, but it's not nothing: even a small, well-sited array can meaningfully reduce the electricity you buy from the grid. The question is whether it makes more sense to use the extension roof alongside panels on the main roof, rather than instead of it. Many homeowners do exactly that — fitting a larger array across both surfaces. If you're thinking along those lines, our guide to solar panels on two different roofs explains how split arrays work in practice.
Are solar panels on a flat extension roof worth it?
Honestly, it depends on your specific situation — and anyone who tells you otherwise without surveying your roof first isn't being straight with you.
The case for an extension roof installation is strongest when: the main roof is genuinely unsuitable (north-facing, heavily shaded, or in poor condition); the extension roof faces south or can be configured to do so; and the structural check comes back clear. In that scenario, the extension roof may be the best — or only — viable surface on the property.
The case is weaker when: the extension roof is small and shaded; the main roof is perfectly capable but you're tempted to avoid the height; or you're trying to add a tiny number of panels that won't move the needle on your bills.
One genuine financial positive worth flagging: solar panels currently attract 0% VAT under HMRC VAT Notice 708/6, a relief that runs until 31 March 2027. That's a 20% saving on the whole installed cost — panels, mounting frames, labour, inverter — compared to a standard home improvement. After March 2027, the rate is expected to revert to 5%, so it's worth factoring into your timing. For more on making the most of this before the deadline, see our guide on 0% VAT on solar panels and the March 2027 deadline.
When solar panels on an extension roof aren't worth it
Being straight about this matters. Skip the extension roof if:
- The flat roof is in poor condition (old, cracked felt or a damaged membrane needs fixing before adding panels — extra cost and disruption).
- The roof faces north or is significantly shaded for most of the day.
- The structural survey flags that the roof joists can't take the ballast load without costly reinforcement.
- The usable area is so small that you could only fit two or three panels — the fixed costs of wiring, inverter, and scaffolding make a tiny system poor value.
In some of these cases, a ground-mounted array in the garden may actually be a better answer than either roof surface. That's covered in our guide to ground-mounted solar panels when your roof can't do the job.
What to look for in a quote for extension roof solar
Any quote for an extension roof installation — especially a flat one — should include a structural assessment, confirmation of the mounting system type (ballasted, mechanically fixed, or A-frame), and clear details on how the cables will be run into the property. If a company wants to skip the site survey or gives you a price without looking at the roof, that's a red flag.
MCS certification is non-negotiable: it's required for your system to qualify for the Smart Export Guarantee, which pays you for surplus electricity you export to the grid. You can check whether an installer is MCS-certified at mcscertified.com.
Getting at least two or three quotes from different installers is the best way to sense-check both the technical approach and the price. FairSolar connects homeowners with vetted, MCS-certified local installers — use our free quote form and you'll hear back from installers who know what a flat or pitched extension roof installation actually involves.
Frequently asked questions
Can I put solar panels on my extension roof?
Yes, in most cases. Both flat and pitched extension roofs can accommodate solar panels. The main checks are orientation (south-facing is best, but flat roofs let you choose the angle), structural load capacity, and whether the roof surface is in good enough condition. A site survey from an MCS-certified installer will confirm whether your specific roof is suitable.
Do I need planning permission for solar panels on an extension?
Usually not. In England, solar panels on an extension roof are permitted development under Part 14 of the GPDO, provided panels don't protrude more than 200mm from a pitched roof slope (or 600mm above a flat roof surface) and don't rise above the highest part of the main roof. Listed buildings and some conservation area situations are exceptions — check with your local planning authority if either applies to you.
Are solar panels on a flat extension roof worth it?
They can be, but it depends on the roof's size, orientation, structural condition, and how it compares to the main roof. A properly tilted south-facing flat roof array produces close to the same output as an equivalent pitched installation. The honest answer is: get a survey, check the structural load, and compare the cost with what you'd get from the main roof before committing.
How many solar panels can fit on a single-storey extension roof?
That depends on the usable area. A typical single-storey rear extension roof might fit 4–8 panels (roughly 1.5–3 kWp), once spacing for tilt frames and edge clearances is accounted for. A small system can still reduce your grid electricity use meaningfully, but if the roof area is very limited it may be worth combining with panels on the main roof rather than using the extension alone.
Is it cheaper to install solar on an extension roof than on the main roof?
Not necessarily. A flat roof installation costs a little more than a standard pitched roof job because of the mounting frames — typically an extra £500–£1,500 depending on system size and mounting type. That said, an extension roof is often at a lower height than the main roof, which can reduce scaffolding costs. Get itemised quotes so you can compare like for like.