Installing a roof window costs between £800 and £2,500, while a skylight typically costs £400 to £1,200
If you want to let light into a loft conversion, a dark hallway, or an upstairs room, you have two main choices: a roof window or a skylight. The price difference is significant and driven by what each product does.
The average installed cost for a basic roof window (supply and fit) is £1,200, while a skylight averages £700 (Checkatrade 2026 price guide). These figures exclude structural alterations such as cutting roof joists or adding a new frame, which can add £500 to £1,500.
Why the gap? A roof window comes with an integrated flashing kit built to seal against rain and wind, and it is typically certified for walk-on loads during maintenance. A skylight is often a simpler, fixed panel that sits on top of the roof surface and uses a separate flashing system that may not be as solid. The Energy Saving Trust notes that the extra manufacturing and certification for roof windows pushes their price higher (Energy Saving Trust, 2026).
The U-value threshold for a compliant roof window is 1.2 W/m²K or lower
Building regulations in England and Wales set a minimum standard for how much heat a roof window can lose. Under Part L 2026, any new or replacement roof window must have a U-value of no more than 1.2 W/m²K (GOV.UK Approved Document L, 2026). A U-value measures how easily heat passes through the window; a lower number means better insulation.
Premium triple-glazed roof windows achieve U-values of 0.8 to 1.0 W/m²K, while standard double-glazed skylights typically achieve 1.1 to 1.4 W/m²K. The Microgeneration Certification Scheme product database confirms that many skylights fall at the higher end of that range (MCS product database, 2026).
If you are fitting a skylight into an existing roof without replacing the frame, the overall U-value may exceed 1.2 W/m²K because the old frame and seal are less effective. In that case, you may not meet building regulations unless you upgrade the frame as well.
A roof window saves 15–20% more energy per square metre than a fixed skylight
The energy performance difference between a roof window and a skylight is not huge in absolute terms, but it adds up over the life of the product. For a typical 0.8 metre by 1.2 metre opening, a roof window loses 45 to 55 kWh of heat per year, while a skylight of the same size loses 60 to 75 kWh per year (Energy Saving Trust field trial data, 2025/2026). That is a 15 to 20% reduction in heat loss per square metre.
The difference comes from the roof window’s better thermal break in the frame and its integrated gasket system, which reduces air leakage. At 2026 energy prices, this saving translates to roughly £8 to £12 per year on heating bills, based on Ofgem’s typical domestic consumption values (Ofgem, 2026). Over a 20-year lifespan, that is £160 to £240 saved.
Quick numbers roof window vs skylight comparison table
| Feature | Roof window | Skylight |
|---|---|---|
| Average installed cost (supply and fit) | £1,200 | £700 |
| U-value (W/m²K) | 0.8–1.2 | 1.1–1.4 |
| Annual heat loss (kWh for 0.96m²) | 45–55 | 60–75 |
| Opening mechanism | Manual or electric | Fixed (no opening) |
| Maximum size without structural support | 1.2m x 1.5m | 1.0m x 1.2m |
| Warranty length (typical) | 10–15 years | 5–10 years |
| MCS eligibility | Yes | No |
Sources: MCS register, 2026; Checkatrade, 2026; Energy Saving Trust product database, 2026.
The direct answer a roof window is the better choice for energy efficiency and usability, but a skylight is cheaper for purely decorative light
For homeowners who want to open the window for ventilation and meet modern insulation standards, a roof window is the correct product. It opens (manually or electrically), has a certified U-value below 1.2 W/m²K, and qualifies for MCS-linked grants if needed. For homeowners who only want overhead light in a room that already has ventilation, a skylight is sufficient and costs less.
The term “roof window” is often used interchangeably with “skylight” in retail and online listings, but the technical difference is the opening mechanism and the certified U-value. The Glass and Glazing Federation advises checking the product specification before buying (GGF member advice, 2026). If the product does not state a U-value or an opening mechanism, treat it as a skylight.
FENSA technical guidance confirms that a roof window is defined as a window that opens and is installed in the roof plane, while a skylight is a fixed glazing panel (FENSA technical guidance, 2026).
You must use an MCS-certified installer for any roof window that will be part of a heat-pump or solar grant scheme
If the roof window is being installed alongside a heat pump or solar PV system to improve insulation as part of the Boiler Upgrade Scheme, the installer must be MCS certified. The Microgeneration Certification Scheme sets standards for renewable energy installations, and the grant requires that all associated work meets its criteria (GOV.UK Boiler Upgrade Scheme guidance, 2026).
For a standalone roof window replacement, FENSA registration or TrustMark certification is sufficient and cheaper. FENSA allows installers to self-certify compliance with building regulations, while TrustMark covers general competency and consumer protection. Gas Safe engineers are not automatically qualified for roof window installation, always check for FENSA or MCS before hiring (Gas Safe Register, 2026).
If you are replacing a roof window as part of a larger energy efficiency upgrade, read our guide to the Boiler Upgrade Scheme and insulation requirements.
How to verify a roof window installer is properly certified
Before you pay a deposit, run three checks. First, if the work is linked to a heat pump or solar grant, search the MCS installer directory at mcscertified.com (MCS installer search, 2026). Second, for building control compliance, check the FENSA register at fensa.org.uk (FENSA search, 2026). Third, for general competency and consumer protection, verify TrustMark registration at trustmark.org.uk (TrustMark, 2026).
After installation, the installer must provide a certificate of compliance. This document is required when selling the house to prove the work meets building regulations. Without it, your solicitor may flag the installation as unregistered, which can delay a sale or require retrospective certification at your cost.
If you are combining a roof window with other window work, see our guide to window replacement and building regulations compliance.
Frequently Asked Questions
A roof window has an integrated flashing kit for weather sealing and is certified for walk-on loads, while a skylight is a simpler fixed panel on the roof surface. The Energy Saving Trust notes that extra manufacturing and certification push roof window prices higher (Energy Saving Trust, 2026).
A basic roof window costs £800 to £2,500 installed, with an average of £1,200 (Checkatrade 2026 price guide). This excludes structural work like cutting joists, which adds £500 to £1,500.
Building regulations Part L 2026 require a U-value of no more than 1.2 W/m²K for new or replacement roof windows (GOV.UK Approved Document L, 2026). A lower number means better insulation.
Yes, skylights typically cost £400 to £1,200 installed, averaging £700, while roof windows average £1,200. The price gap is driven by the integrated flashing and walk-on certification of roof windows (Energy Saving Trust, 2026).
Standard double-glazed skylights achieve U-values of 1.1 to 1.4 W/m²K, so many meet the 1.2 W/m²K limit, but some fall at the higher end. The MCS product database confirms this range (MCS product database, 2026).
Replacing a roof window like-for-like usually does not need planning permission, but if you change the size or position, you may need approval. Always check with your local planning authority, as rules vary across the UK.
