Part L 2026 for self-builders

First: which standard applies to you

The dates decide it, and they are strict — the timeline has the detail:

• an application submitted before 24 March 2027, with work commenced before 24 March 2028, stays under the previous standards
• everything else is caught

For a self-build this matters more than for a volume developer, because self-build programmes slip. If you are relying on transitional protection, the commencement date is the one to watch — an application filed in good time is worth nothing if the ground is not broken by March 2028.

If your project straddles the boundary, design to the new standard anyway. Building to 2027 rules on a 2026 application costs you very little and removes the risk entirely.

Your number

Ground floor area ÷ 11.4 = target kWp. That is 40% of the ground floor area at 0.22 kWp/m², which is Approved Document L’s equation 5.1 rearranged — the full explanation is on the 40% rule.

The requirement is an annual output at least equal to that notional array on a south-east to south-west roof at 45°, unshaded. Which means the design decisions below are not cosmetic — they change how much capacity you need to hit the same target.

Our roof area calculator will convert usable area into capacity and generation for your location and orientation.

The decisions that are cheap now and dear later

Sizing beyond the minimum

The target is a floor, not a ceiling, and the economics at build stage favour going past it.

Much of an installation’s cost is fixed — scaffolding, mobilisation, the electrical connection, the certification — so the cost per kW falls as the array grows. That is visible in the official statistics: the median cost per kW in the 4–10 kW band is below the 0–4 kW band, and the 10–50 kW band is lower again. See cost per kW.

Two constraints to design around, rather than discover:

The G98 threshold. G98 covers registered capacity up to 3.68 kW single-phase. Most self-builds will exceed that, which means a G99 application — a process to start early, not at second fix. If your plot has or can have three-phase, the equivalent limit is 11.04 kW.

Export capacity. Consultation responses to the standard raised “the difficulty of obtaining connections for export capacity”, and the impact assessment concedes some sites may not be able to export at all. Ask your network operator early; the answer shapes whether a large array makes sense.

If the roof genuinely cannot get there

Approved Document L has a route, and it is documentary rather than a shrug.

First, maximise. Paragraph 5.76 expects design changes — higher performance panels, alternative orientations, repositioning roof vents and architectural features — before a shortfall is accepted.

Then, evidence it. Under Appendix B you need:

• roof diagrams with and without the proposed panels, showing the design restriction
• a statement setting out why a lower provision is being installed and why the design cannot change
• calculations supporting it, signed by a suitably qualified person — a qualified On Construction Domestic Energy Assessor is named

And it gets recorded. Any array falling short of the standard is highlighted in the BREL report under paragraph 5.78. On your own house, that is a document a future buyer will see.

The exceptional-circumstances floor is a roof that cannot support an array generating 720 kWh a year — roughly two panels’ worth. It is a genuinely high bar to fall under.

Certification, and the trap for self-installers

Two separate things, and self-builders regularly conflate them.

Building Regulations certification of the electrical work. Notifiable, and certified either by a registered competent person or by a building control body.

MCS certification of the installation. Not a building regulations requirement at all — but it is what suppliers ask for before paying you SEG export income.

MCS certifies installations carried out by MCS-registered installers. So a self-installed array, however competently built, is likely to leave you outside the export payment system. If export income is part of your plan, use an MCS-registered installer and get the certificate at handover.

VAT, and one date

Installation of solar in residential accommodation is zero-rated until 31 March 2027, reverting to 5% from 1 April 2027 under HMRC’s guidance. Note also that supplying materials without installation is standard-rated — relevant if you were planning to buy panels and have someone fit them separately.

Self-build has its own VAT reclaim scheme with its own rules, and that is a question for HMRC or your accountant. We mention it only so it does not get forgotten.

A sensible order of operations

1. Establish which standard applies from your application and commencement dates.
2. Do the ÷ 11.4 arithmetic at design stage, before the roof is drawn.
3. Orient and clear the roof plane for the array.
4. Ask the network operator early about capacity and export.
5. Run the conduit for storage and an EV point whether or not you buy them now.
6. Use an MCS-registered installer if you want export income.
7. Collect the certificates — building regulations, MCS, DNO — and keep them with the house.