Yes, you can usually add solar panels to a home that already has them, or add a first array to a property with a little roof space spare. What decides the route is not the roof. It is your inverter’s spare capacity, the grid connection limit and whether the new panels can sit alongside the old ones without dragging output down. This guide covers each of those checks, the costs worth knowing, the paperwork people forget, and where a battery fits in.
TL;DR
- Extending a system comes down to three questions: does your inverter have spare capacity, will new panels match the old, and does the total stay under the 3.68kW per phase G98 limit?
- Going over that limit generally means a G99 application to your network operator. Tell the operator about any increase in capacity, even if you stay under it.
- Adding panels to an existing system tends to be cheaper than a first install, especially if scaffolding is already up for other work.
- Plug-in solar is a separate, tightly regulated category. Roof arrays still need a professional, MCS-certified installer.
- If you are adding panels anyway, price a battery in the same visit.
Table of Contents
- The three ways to expand a solar system
- Check your grid connection before you buy panels
- What it costs to add solar panels
- Planning permission and building regulations
- Can you add panels yourself? Plug-in solar explained
- Add a battery while you are at it
- Our perspective: size for the home, not the roof
- Getting a quote from us
- FAQ
- Sources
The three ways to expand a solar system
People searching for how to expand a solar system are usually picturing one thing: more panels on the roof. In practice there are three routes, and the right one depends on what is already installed.
1. Use a spare input on your existing inverter
Many inverters have a second input (a second string) and some headroom. If yours does, new panels can be wired to that input. This is often the cheapest route because the expensive electronics stay put. The new panels should not share a string with old ones of a different type, because mismatched panels pull each other down to the weakest performer.
2. Add a second inverter for the new panels
If the current inverter is full, or the old panels are an awkward match, a second inverter with its own string keeps the two arrays independent. You pay for extra hardware, but the old system carries on exactly as before.
3. Replace the inverter, or the whole system
Older panels and inverters can be a poor basis for expansion. A new hybrid inverter, which can also run a battery, is sometimes the sensible move when the old unit is near the end of its life or has no spare capacity. We cover the main options in our guide to solar inverter types and G98/G99.
A site survey is the only way to know which route applies to you. An installer needs to see the inverter’s data plate, the string layout, the roof and the consumer unit.
Check your grid connection before you buy panels
This is the step most homeowners overlook, and the one that causes real problems later.
The 3.68kW line: G98 versus G99
Under the connection rules, a single-phase home system up to 3.68kW per phase is generally connected under G98, which is a notify-after-install process. Above that, you generally need to apply to your distribution network operator (DNO) under G99 before installing, and the DNO decides whether local grid capacity allows it. That is slower and more involved. Ofgem is the regulator for the networks, and our own explainer on the G98 versus G99 threshold walks through how it applies to homes.
Here is the catch with expansion. If your existing system is already close to 3.68kW, adding even a few panels can tip the total over the line and turn a simple job into a G99 application.
Tell your DNO about the increase
An expansion changes your system’s registered capacity, so your DNO paperwork needs updating. If you skip it, you may be in breach of your grid connection terms, and that can cause trouble with warranties, insurance and export payments such as the Smart Export Guarantee. A competent installer handles this notification as part of the job. If yours does not mention it, ask.
If you want to understand how export payments work once you have more panels, see our guide to the Smart Export Guarantee. Smart meters also matter for export measurement, covered in how smart meters work with solar.
What it costs to add solar panels
We do not publish fixed prices for installations, because the spread between jobs is wide. What we can say is where the savings come from, using published figures.
As a benchmark for a new system, the Energy Saving Trust’s figures put a typical 4.5kWp installation in Great Britain at around £7,100 as of April 2024. That was a first-time installation, and the same guidance notes that adding to an existing system is generally cheaper, particularly if you can use scaffolding that is already up.
Scaffolding matters because it is a large fixed cost. The Energy Saving Trust, quoting Checkatrade, gives an average of £875 a month for a semi-detached house, and suggests considering solar if you are already doing roof work. If you are re-roofing or repairing anyway, that is the cheapest time to add panels.
The main cost variables on an expansion:
| Factor | Why it changes the price |
|---|---|
| Spare inverter capacity | Using an existing input is cheaper than a second or replacement inverter |
| Panel matching | Old panel models can be hard to match, pushing you to a separate string |
| Access and scaffolding | A fixed cost, best shared with other roof work |
| Grid application | A G99 application adds time and admin compared with G98 |
| Battery at the same time | Shares access, wiring and consumer unit work |
Treat any per-panel price you see online with caution. Without knowing your inverter and roof, it is a guess.
Planning permission and building regulations
In England, rooftop solar is generally permitted development. The usual conditions are that panels do not protrude more than 0.2 metres from the roof slope and stay below the highest point of the roof, excluding the chimney. Listed buildings and conservation areas have special rules, and a wall fronting a highway may need permission. Rules in Wales, Scotland and Northern Ireland differ, so check locally.
If you are unsure, ask your local planning authority or apply for a Lawful Development Certificate. We have written a fuller guide: do you need planning permission for solar, plus a wider walkthrough of how solar planning works.
Planning is only half of it. Even when permission is not needed, the work is still subject to building regulations. Your installer should cover the structural and electrical side. If your roof is older, see our notes on common roof structure challenges, and if part of the roof is shaded, read about shading and output before deciding where the new panels go.
Can you add panels yourself? Plug-in solar explained
The government has been moving towards allowing small plug-in solar devices, with a consultation on amending the Plugs and Sockets Regulations and an interim product specification. These are small units, a panel or two on a wall, shed or garden, connected through a suitable outdoor socket, without a battery.
That is not the same as extending a roof system. A standard fixed array and any battery storage still need professional installation, with MCS certification and wiring to BS 7671, if you want the system to be compliant and eligible for export payments. Treat DIY work on a roof array as technically complex and legally risky. A plug-in unit can suit a renter or a flat-dweller with a sunny wall. It will not replace a properly designed roof system for a family home.
Add a battery while you are at it
If you already have panels, you may be exporting a lot of midday electricity for a modest return and buying it back in the evening. The Energy Saving Trust explains that a battery stores surplus solar generation so you can use it later, and can be paired with a time-of-use tariff. Its figures put a 5kWh battery at around £4,600 and a 7.5kWh unit at around £6,500, and it notes savings can be significant but will not always be enough on their own.
Doing both at once is generally more efficient. The same site access, wiring and consumer unit work serve both jobs. Existing systems can often be retrofitted with a battery later, but a combined visit avoids paying for the groundwork twice.
Before choosing a size, use your own data. Our guide to sizing a battery from 12 months of meter data shows how to avoid paying for capacity you will never fill. If you want to know whether it pays back, read when home batteries pay back. Households on cheap overnight rates can also look at off-peak battery charging.
More generation also makes the case for using the power well. If you drive an electric car, an extra array pairs naturally with a home charger, and a quick look at EV charge point options shows what is on offer for your car. We cover the practical side of solar and charger pairing in our Hampshire guide.
Our perspective: size for the home, not the roof
We install solar, batteries and EV chargers across Dorset, Hampshire and Devon, and the expansion enquiries we see follow a pattern. Someone has a system that was sized to a minimum, often when the house was built, and the household has since added an electric car or a heat pump. The instinct is to cover every spare roof tile.
Our advice is to start with the inverter and the grid connection, not the roof. A few extra panels that stay under the G98 limit and use a spare input can be a neat, low-cost job. Pushing past 3.68kW can still be right, but it should be a decision made with the G99 process in mind, not a surprise halfway through.
We also think warranty paperwork deserves as much attention as the panels. Before you sign anything, check how the new work affects the existing guarantees, and read our three warranty checks. We hold MCS and NICEIC accreditation, and we would rather tell you that a smaller extension is enough than sell you a bigger one.
— Simon
Getting a quote from us
We cover solar installations and battery storage for homes and businesses in Dorset, Hampshire and Devon, from a first array to an extension of an older system. Send us your inverter model, a photo of your consumer unit and your latest meter readings, and we can tell you which of the three routes fits. Once the system is running, our Solar & Battery Service & Maintenance Plan costs £175 per year. You can start from our main site.
FAQ
Can I add more solar panels to my existing system?
Usually, yes. The deciding factors are spare inverter capacity, panel compatibility and whether your total stays within the G98 limit of 3.68kW per phase. An installer will check these on a survey before recommending an extension, a second inverter or a replacement.
Do I need to tell my DNO if I add panels?
Yes. An increase in capacity changes your registered connection, and not notifying the network operator can cause compliance issues. It can also affect warranties, insurance and export payments. If the new total goes over 3.68kW, you will generally need approval first through a G99 application.
Is it cheaper to add panels than to install a new system?
Generally, yes, as you already have the inverter and some of the wiring in place. Costs fall further if scaffolding is already up for other roof work. Prices still vary with the job, so a survey-based quote beats any online per-panel figure.
Can I fit extra solar panels myself?
Not for a roof array. Fixed systems should be installed by an MCS-certified professional to meet BS 7671 and keep export eligibility. Only the new, tightly defined plug-in solar category is moving towards being suitable for householders.
Sources
- Battery storage, Energy Saving Trust
- Energy saving upgrades for home renovation, Energy Saving Trust
- Ofgem
- Solar panels and permitted development, Wirral Council
- Building regulations and renewables guidance, Bedford Borough Council
- Plug-in solar: regulatory amendment and interim product specification, GOV.UK
