Loft Conversion Windows: Velux vs Dormer vs Juliet Balcony — London Guide

Every loft conversion needs windows for light, ventilation, and — where applicable — emergency egress. London planners and Building Control both have opinions on which type is appropriate, and the structural reality of your roof limits the options further.

The four main categories are: roof lights (Velux-style), dormer windows, Juliet balconies, and fixed skylights. Each sits in a different position on the cost, planning, and light-output spectrum.

Velux is a brand name that has become the generic term for roof lights — flush-fitting windows that sit within the plane of the slope. Other manufacturers include FAKRO and Keylite. Roof lights are by far the most common choice for London loft conversions because they usually fall within permitted development, cost less, and preserve the roofline silhouette that many conservation areas and Article 4 directions require.

A standard Velux roof light costs £400–£900 for the unit, plus £300–£600 installation per window depending on access and roof covering. A typical bedroom conversion needs two to four roof lights to meet Building Regulations daylight requirements (minimum 10% of floor area as glazing for habitable rooms).

Advantages: usually permitted development on principal elevations, lowest cost, fastest to install, good ventilation when opening types are used, wide product range including remote-controlled blackout blinds, rain sensors, and solar operation.

Disadvantages: only light comes from the slope — no vertical view when standing up, can feel like looking at the sky rather than the street, limited headroom unless the loft already has good ridge height, heat gain in south-facing slopes can be significant without solar control glazing.

For conservation areas and listed buildings: even roof lights may require planning permission if they are on the principal (street-facing) elevation. Always check with the local planning authority before ordering.

Building Regulations note: if the room is a habitable space, at least one opening roof light per habitable room is required for ventilation compliance (purge ventilation). Fixed skylights alone will not satisfy this requirement — you would need additional trickle vents or mechanical ventilation.

A dormer is a structural projection from the roof slope that creates a vertical-faced window. It is the most significant light and space upgrade available for a loft conversion — adding both headroom and usable floor area beside the window itself.

Cost range: a single rear flat-roof dormer runs from £8,000–£15,000 in London, including structural work, roofing, rendering or cladding, and windows. A full-width rear dormer — which maximises the loft footprint — costs £15,000–£30,000 depending on width, specification, and glazing choices.

Advantages: vertical glazing provides a proper view; standing headroom close to the window; more usable floor area per square metre of roof converted; can accommodate French doors or Juliet-style railings as part of the design.

Disadvantages: structural work required (new timbers, beam, lintel); planning permission required for side and front elevations in most London boroughs; rear dormers may fall under permitted development but this depends on the property type, borough, and whether it is in a conservation area; more disruptive installation (scaffold needed, roof open for longer); higher cost.

Permitted Development for rear dormers: a rear dormer can fall under PD on a house (not a flat) if it does not exceed the highest point of the existing roof, does not extend beyond the outer face of the original side wall, and does not add more than 40 cubic metres (terraced) or 50 cubic metres (detached/semi) of additional roof space. Many London boroughs have Article 4 directions that remove PD rights — check with the council before proceeding.

Hip-to-gable conversions: on a hipped roof property (one with a sloping end rather than a flat gable wall), filling the hip to create a full gable end is permitted development in the same conditions. Combined with a rear dormer, this gives the maximum possible loft footprint.

A Juliet balcony is a full-height glazed door that opens to a flush safety railing — there is no external platform to stand on. It creates a dramatic sense of space and natural light but does not add usable external area.

Cost: £2,500–£6,000 including structural opening, door unit, and railing system. The structural lintel required to form the opening is a significant part of the cost.

Planning: Juliet balconies almost always require planning permission because they involve a new vertical opening in the roof structure (usually as part of a dormer) and the railing must be visible from the street or neighbouring properties. In conservation areas, they are frequently refused.

Where they work well: rear elevations where planning permission is being sought for a dormer anyway; contemporary architectural styles where the glazed railing is appropriate; rooms that benefit from a view but where external space is not needed; south-facing rear elevations where the full-height opening creates a dramatic light effect.

Where they are problematic: conservation areas and listed buildings; north-facing rooms where opening the doors provides little benefit; properties where overlooking neighbouring gardens would cause a planning objection; narrow loft rooms where the door reduces usable floor area.

Fixed skylights do not open. They are the simplest structural intervention — no hinge mechanism, no drainage around an opening — but they have a significant limitation: they cannot be used alone to satisfy Building Regulations ventilation requirements for habitable rooms.

Cost: £600–£2,000 for a quality unit, plus installation.

Best uses: bathrooms and en-suites (where mechanical extract ventilation provides ventilation compliance), stairwells (light without needing ventilation), landing areas, or in combination with opening roof lights that provide the required purge ventilation.

Flat roof or walk-on rooflights: where the loft extension involves a flat roof section, a walk-on rooflight can be a striking design feature. Structural glazing units with slip-resistant coating cost from £3,000–£8,000 plus structural and waterproofing work. They require planning permission in most cases.

Beyond the type of window, the glazing specification significantly affects comfort, energy performance, and cost.

Double vs triple glazing: triple glazing offers better thermal performance (lower U-value) and better sound attenuation — useful in London where road noise and aircraft noise are common. The premium over double glazing has fallen and is typically 15–25% for the unit. Building Regulations set maximum U-values for roof windows — triple glazing almost always complies comfortably; double glazing must be specified carefully to meet the 1.4 W/m²K limit for roof lights.

Solar control glazing: on south and west-facing slopes, untreated glass allows significant heat gain in summer. Solar control glass has a coating that reduces this gain. In London, south-facing loft bedrooms without solar control glazing regularly reach 35°C+ in July — seriously affecting habitability and potentially requiring mechanical cooling. The product is strongly recommended for any south or west-facing installation.

Tinted and self-cleaning glass: tinted glazing reduces glare; self-cleaning (photocatalytic) coatings reduce maintenance on hard-to-reach roof lights. Both add cost (10–20%) but are worth considering for high-slope or access-difficult positions.

Blackout blinds: any bedroom roof light needs a blackout blind to be usable for sleep in London summers where it stays light until 10pm. Velux and FAKRO both offer integrated blind systems — budget £150–£400 per window for a quality blackout blind with side channels to prevent light leakage.

The best choice depends on your roof type, planning situation, budget, and the use of the room.

For a simple one-bedroom loft conversion in a Victorian terrace: rear roof lights (Velux-style) are usually the right starting point. Two to three rear-facing roof lights will provide sufficient light, meet ventilation requirements, and — if the rear elevation is not in a conservation area — will almost certainly be permitted development.

For a double-bedroom or large master suite: a full-width rear dormer is the right structural move if planning allows. The additional floor area and vertical glazing transform the feel of the space compared with roof lights alone.

For a hipped-roof property (common in 1930s semi-detached houses): consider a hip-to-gable conversion combined with a rear dormer. This gives a full gable wall on the side (with a standard window) and a full-width rear dormer — maximising space and light.

For a listed building or conservation area: roof lights (not dormers) are almost always the only realistic option. Choose a conservation specification unit (lower profile, dark grey or black frame) to meet the planning authority's design expectations.

For a bathroom or en-suite: fixed skylights work if combined with mechanical extract ventilation. Opening roof lights are simpler and more cost-effective.

For a south or west-facing room used as a bedroom: solar control glazing and quality blackout blinds are not optional — they are essential for habitability.