The costs that are not in the headline price

Access and scaffolding

Usually the largest non-panel item, and the one most sensitive to your specific house.

What moves it: height, roof pitch, ground conditions, a conservatory or extension in the way, restricted side access, and how long the scaffold must stand.

The licence. Where scaffolding stands on a public footway or road, the local authority requires a licence and charges for it. On a terraced street with no front garden this is routine, and it is a cost somebody pays.

The question to ask: is scaffolding included, for how long, and who pays if it has to stay up longer or come back?

Electrical work the survey turns up

A solar installation adds a generating circuit to your house, and the existing installation has to be fit to receive it.

Common findings:

• No spare way in the consumer unit
• Older protective devices without modern residual current protection
• Earthing arrangements that need improvement
• A long cable run from a loft inverter to the board

None of this is the installer inventing work. It is what happens when new equipment meets a board installed in 1998. What matters is when you find out — a survey that identifies it produces a priced quote, and a discovery on installation day produces a phone call while the scaffolding is up.

The electrical work in a solar installation is normally notifiable under the Building Regulations, which is why it comes with a compliance certificate — see building regulations. That certificate is not optional paperwork; it is what your buyer’s conveyancer will ask for years later.

The roof underneath

The most expensive avoidable mistake in domestic solar, and it is entirely a question of sequence.

If the covering is near the end of its life, re-roof first. Doing it afterwards means paying to remove the array, scaffolding a second time, storing panels, refitting, and re-testing. The panels do not care; your wallet does.

Ask the installer what they found. A structural assessment should have been done — see structural surveys for what that involves and what it should have covered.

Ask who pays for broken tiles. Some breakage is normal on brittle coverings. Establishing responsibility before work starts costs nothing; establishing it afterwards costs an argument.

Network constraints

Two ways the network operator can change your economics.

Export limitation. Where the local network cannot accept full export, the operator may require the installation to be limited — see export limiting. It affects what you can sell, not what you can use.

G99 rather than G98. Above 16 A per phase — 3.68 kW single-phase — you are in the G99 process: apply, wait for a decision, then install. That is time rather than a large fee for a domestic system, but it is a real difference in the programme, and a refusal or a conditional offer changes the design. What happens if the answer is no is covered in when your DNO refuses.

Replacement, and the honest gap in what we know

The inverter is the component most likely to need replacing first. We are not going to tell you when, because we have no UK failure-rate data we trust and manufacturers’ warranty terms are a commercial promise rather than an observed service life.

What we will say:

• Budget for at least one replacement across the life of the panels.
• Ask what the warranty term is, for the inverter specifically rather than the system.
• Ask what a replacement costs today — the installer knows, and the answer tells you something about how they think about the long term.
• Panels themselves typically carry much longer performance warranties than inverters, and degrade rather than fail.

VAT changes on 1 April 2027

HMRC’s VAT Notice 708/6 is explicit that from 1 April 2027 installations of energy-saving materials “will revert to the reduced rate of VAT of 5%”.

For a system at the 2025/26 median — about £6,400 for 4 kWp — that is roughly £320.

Two practical points. If your installation will straddle that date, ask the installer how they will treat it. And note that the relief applies to installation: supplying materials without installing them is standard-rated, so a self-managed job does not qualify.

Smaller items that catch people

• Bird protection — mesh around the array, often quoted as an extra, worth having where there are pigeons
• Tree work — the shading you can remove is cheaper than the optimisers that manage it
• A meter change — you may need an export-capable meter for SEG, which your supplier arranges
• Insurance — tell your insurer; the cost of asking is nothing
• Loft boarding or access where the inverter goes somewhere awkward
• Making good — cable entry points, decoration where the run passes through the house

The one thing worth doing before you sign

Ask, in writing: “What would change this price after survey, and by roughly how much?”

An installer who answers that properly has done a real survey and is telling you where the uncertainty is. An installer who says nothing can change is either unusually confident or has not looked, and you will find out which on the day.