Rear Extension Planning: Windows, Doors and Roof Choices

Start with How You Want to Use the Space

Before you fix a single dimension on a drawing, spend an evening thinking about a typical Saturday. Where does the light land at 4pm in June? Will someone be cooking while others sit outside? Do you need a clear route from the kitchen island to the patio with a tray of drinks? These small questions shape glazing far more than any style board.

Most homeowners arrive with a picture in mind: a wall of glass, sliding doors, a lantern roof. That instinct is sound, but it needs testing. A rear extension works hardest when it solves three problems at once — daylight, connection to the garden, and a comfortable temperature in February as well as July.

It helps to split your plans into three coordinated decisions: the glazing format (windows and doors), the opening sizes, and the roof. Get them talking to each other and the result feels effortless. Get them wrong and you end up with a room that is either gloomy at one end or baking hot at the other.

Choosing Between Sliding, Bifold and French Doors

The door set is the emotional heart of the project, so it is worth understanding the trade-offs rather than following fashion.

• Sliding doors need less clear wall space than bifolds, glide on a single track and give you wide, uninterrupted glass. A two or three-panel slider is often the sweet spot for a typical 3m to 4m opening.
• Bifold doors fold right back and open up the whole width, which is wonderful for entertaining. They do need somewhere for the stacked panels to sit, and the frame lines are busier.
• French doors remain the most economical choice and suit period homes. A pair at 1.5m to 1.8m wide with sidelights either side can feel surprisingly generous.

Think about thresholds too. A flush or level threshold — where the internal floor meets the patio with minimal step — transforms how the garden feels part of the room. It costs a little more and demands careful drainage detailing, but it is usually money well spent. If you have young children or a step-free requirement, this detail matters even more.

Getting Window Heights and Sills Right

Awkward proportions are the most common regret. A window set too high leaves a dead band of wall below and cuts off the garden when you are seated. Wherever possible, keep sill heights at around 750mm to 900mm in living areas so the view works from a sofa or dining chair.

Full-height glazing looks dramatic but needs managing. In a west-facing extension it can mean low winter sun straight into the eyes and overheating in summer. Consider a mix: full-height panels facing the best view, with a solid pier or a lower sill where you need shade, a television wall, or a run of kitchen units.

Also plan for ventilation. Trickle vents in the frames are now standard, but a couple of opening lights still make a real difference on a still August evening. Roof lights that open are another option, and they help hot air escape at the highest point of the room.

Roof Options: Lanterns, Flat Roof Lights and Pitched Designs

The roof does more for the feel of an extension than almost anything else, and it is where budgets quietly drift.

• Pitched roof with roof windows — the most cost-effective way to add overhead light, and the shape suits traditional properties. Insulation is straightforward and performance is excellent.
• Flat roof with roof lights — clean, modern lines and simple to build. Choose high-spec double or triple glazed units with a low U-value and a solar control coating to keep summer heat out.
• Roof lantern — brilliant over a kitchen or dining zone, throwing light deep into the plan. They are the priciest option and need careful flashing and upstand detailing.

Whichever you choose, insist on a properly insulated, thermally broken roof build-up. A cold roof over a warm room is a recipe for condensation, and no amount of glazing will compensate.

Coordinating Structure, Glass and Budget

Every metre of glass affects the steelwork. Wide openings need a structural beam, and a beam means either a deeper ceiling or a hidden steel in the roof void — both of which shrink head height. Sort out the engineering early, because discovering you need a 300mm beam after you have ordered 2.4m doors is an expensive conversation.

As a rough guide, allow for the fact that glazing typically accounts for somewhere between a fifth and a third of the total build cost. If the budget tightens, reduce the number of openings rather than the quality of the glass. One beautifully made, well-insulated slider beats four cheap ones every time.

Finally, think about the garden itself. A patio that sits level with the new floor, planting that softens the boundary, and perhaps an outdoor socket or tap near the doors will complete the connection you were chasing. The extension and the garden should read as one continuous space, and that is achieved through coordination, not by adding more glass.