New Part L for Wales (Onsite generation)

Here are some further details about the changes to Approved Document Part L (Wales), which closely follows the Future Homes Standard targets which apply to England, but are scheduled to come into force three weeks earlier; being applied to new building sites from 4th March 2027.

https://www.gov.wales/sites/default/files/publications/2026-04/building-regulations-approved-document-l-vol-1-2026.pdf

☀️ This post focuses on the new requirement for onsite energy generation.

Onsite generation will become mandatory on all new buildings in Wales, unless there's a good reason not to.

As recently announced in England, PV panels will be required on the vast majority of new buildings.

SAP runs a calculation based on a south east facing PV system covering 40% of the ground floor area, generating 0.22 kWp per m2, at a 45 degree tilt. The output (kWh) is the Functional Requirement target.

New homes need to meet this kWh value. When you consider the specific plot orientation and panel efficiency, the required array size won’t precisely match 40%, but it’s thereabouts.

🏰 If the dwelling has dormers, rooflights or a funky roof design, it may fall short of the PV requirement. When this happens, the PV designer needs to present the max-fit scenario to Building Control for approval. The lower value can then be used both in SAP and onsite.

The Notional Specification (see previous post) copies the Functional PV requirement. So if you can only fit 25% PV coverage, the Notional uses 25% as well. This means you’re not penalised for having lower levels of PV. But on the flip side, your SAP results won’t start showing any improvement until the PV goes beyond 40%.

❌ If the Functional Requirement calculates annual output below 720kWh for a dwelling, this triggers an exemption.. the roof isn’t considered suitable.

🙂 Side note that makes me happy:

The PV Functional Requirement and Notional Specification always match precisely. This isn’t true in the English version, where there are slight differences in the calculations (which my OCD isn't a fan of), so a gold star to Wales for making sure they match. ⭐ 

😬 Side note 2 (that is unnecessarily infuriating):

Part L includes guidance for max-fit PV designs and suitable spacing. The Welsh and English copies are identical, except for the gap to the ridge line. Wales lists 400mm; England 600mm. That one discrepancy means PV designers will need to redraw designs to be suitable for Wales.

Offsite generation

The England (UK) version insists generation must occur within the plot curtilage. The Welsh version allows for offsite generation and/or microgrids, providing:

◾ the generated energy provides an equivalent to the 40% PV rule for all dwellings connected to the offsite supply, and

◾ the generated electricity is directly connected to dwellings, and gives them priority use.

🏨 For apartments, it’s OK for PV to feed a landlord’s supply instead of directly into individual dwellings, providing the residents benefit from the energy in some way. For example, cheaper service charges due to the landlord’s energy bills being lower.