What U-value do roof windows need?

Under Part L, roof windows must achieve a U-value of 1.4 W/m²K or better (same as vertical windows). Most quality double-glazed roof windows achieve 1.2-1.4 W/m²K. Triple glazed units achieve 0.8-1.1 W/m²K. Roof windows lose more heat than vertical windows due to greater exposure.

Do roof windows need planning permission?

Under permitted development, roof windows don't usually need planning permission if they don't protrude more than 150mm beyond the roof plane, are not on a front-facing roof slope in a conservation area, and the property isn't listed. They always need Building Regulations approval.

What size roof window do I need?

Part F requires ventilation openings of at least 1/20th of the room floor area. For loft rooms, the glazed area should ideally be 20% of floor area for good daylight. MK06 (780x1180mm) is the most popular domestic size. Pairs of smaller windows often work better than one large one.

Centre-pivot or top-hung roof window?

Centre-pivot windows rotate around a central hinge and work well at any height. Top-hung windows hinge at the top and open outwards giving an unobstructed view. Top-hung is better at lower positions for safety and ease of use. Electric operation suits high or hard-to-reach positions.

How much does it cost to install a roof window?

Supply and fit for a standard MK06 centre-pivot roof window costs £600-900 including flashing kit. Allow £150-300 extra if the roof structure needs trimming. Electric windows add £200-400. Replacing an existing window is cheaper than installing new as no structural work is needed.

TradeCalculator

Roof Window Calculator

Calculate roof window sizes, costs and natural daylight contribution. Covers centre-pivot and top-hung VELUX-style windows with Part L U-value checking.

Window Size

MK06 is most popular

Opening Type

Centre-pivot most versatile

Glazing

Triple for better insulation

Quantity

Number of roof windows

Roof Pitch

Typical UK pitch 30-45\u00b0

How We Calculate This

This calculator estimates roof window costs including supply, flashing and installation.

The formula

Total = (Unit Price × Opening Multiplier × Glazing Multiplier × Qty) + Flashing + Installation

Electric operation adds ~60% to unit cost
Triple glazing adds ~35% to unit cost
Flashing kit ~\u00a375 per window
Daylight factor considers roof pitch angle

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.