Self-building? Solar decisions start before the roof does

The Future Homes Standard — solar is now mandatory for new builds

The Future Homes Standard (Part L 2026) was published on 13 March 2026 and comes into force on 24 March 2027. Under the new functional requirement (Part L3), solar PV must cover at least 40% of the ground-floor area of a new dwelling. For a typical 3-bed house with a 42.5m² ground floor, that is approximately 17m² of panels — around 3.4 kWp at 425W per module.

The 75% carbon reduction target vs. the 2013 baseline cannot be met without on-site renewable generation, and solar PV is the most cost-effective way to achieve it. An exemption applies only where the minimum annual output of 720 kWh/year genuinely cannot be achieved — for example on a heavily shaded north-facing roof — and must be documented in the compliance file.

Transition: Projects with planning applications submitted before 24 March 2027 may build to the current 2021 Part L standards, provided construction commences by 24 March 2028. If you are at pre-application stage now, factor this into your programme. See our guide to new build solar and the Future Homes Standard for the full regulatory picture.

In-roof vs on-roof — the self-builder's real choice

On a retrofit, in-roof systems cost more because existing tiles must be stripped and relaid. On a new build with no tiles yet, the equation inverts: in-roof panels replace the tile layer, so you save the cost of the tiles they displace. A 33m² Welsh slate roof costs approximately £2,200 in materials alone. On a new build, the net cost of in-roof is often similar to or less than on-roof plus tiling.

The most widely used in-roof system in UK new builds is Viridian Clearline Fusion — accepted by NHBC, LABC, and Premier Guarantee, works with all standard tile profiles, and installs at around £1,600/kWp. The GSE In-Roof system is a budget-accessible alternative from ~£1,400/kWp.

For a higher-specification aesthetic, BIPV tiles (solar tiles that look like conventional roofing) cost more but visually integrate completely:

• Marley SolarTile: ~£2,400/kWp installed, MCS-certified, widely stocked
• GB-Sol PV Slate: ~£2,750/kWp, Welsh-slate aesthetic, 25-year product and weatherproofing warranty, NHBC accepted
• Tesla Solar Roof: Not practically available via a UK order route as of 2026 — a handful of pilot installs only. Do not spec it until domestic availability is confirmed.

Trade-off to know: in-roof panels run 5–10°C warmer than on-roof (no underside air gap), which reduces yield by roughly 5–10%. Panel efficiency is identical; the thermal penalty is the only difference.

See our solar roof tiles guide for a detailed cost and product comparison.

When to specify solar in your build programme

Solar decisions feed into other trades — get them wrong sequentially and you create expensive rework. The key stages:

• Design stage (before Building Regulations application): Your SAP assessor needs the solar PV specification to produce the design-stage calculation. Panel area, orientation, pitch, shading, and inverter efficiency are all inputs. The system size must appear in the compliance file. If you are targeting FHS compliance from 2027, demonstrate ≥40% ground-floor area coverage at this stage.
• Roof framing stage: If using in-roof (Viridian, GSE, GB-Sol), specify a continuous underlay/membrane running to the gutters — this is structural to the in-roof system and expensive to retrofit. Confirm rafter spacing: most kits are designed for 400mm or 600mm centres.
• Roofing stage: In-roof panels go in at this point, before tiles. Agree DC cable routes to the plant room with your electrician before the roof is closed.
• First-fix electrics: Run DC cable from roof void to the inverter location (typically garage or plant room) alongside other cable routes. Submit your G98/G99 DNO notification during this period — lead time can be 6–8 weeks for systems above 3.68 kWp.
• Second-fix / completion: Inverter and generation meter installed by your MCS-certified installer. As-built SAP filed with Building Control; EPC issued.

For in-roof systems, the interface between roofer and solar sub is the biggest coordination risk. Using a specialist who holds both a roofing qualification and MCS certification — or subcontracting the entire roof zone to one trade — eliminates disputed liability if a leak develops at a flashing joint.

VAT — self-builders get a meaningful advantage

Solar panels supplied and installed together by a single VAT-registered MCS installer are currently zero-rated (0% VAT) on residential properties until 31 March 2027, when the rate reverts to 5%. On a typical 4 kWp system this saves around £1,000–£1,400.

Self-builders also have access to the HMRC DIY Housebuilder VAT Reclaim (VAT431NB): if you buy materials separately from a distributor (who may charge standard VAT on goods), you can reclaim that VAT on completion — within 6 months of the completion date.

The simplest route is to use a single supply-and-install MCS contractor while the 0% window is open. This avoids VAT complexity entirely and ensures Building Control, MCS certificates, and SEG eligibility are all handled by one party. Splitting supply from installation creates potential 20% VAT exposure on materials that you then need to reclaim separately.