Glazed rear extension

Glazed and glass-box rear extensions

A glazed rear extension uses full-height sliding doors, structural glass or a glass-box corner so the garden becomes part of the room. They are the most striking rear extensions and the most technical: the glass has to meet Building Regulations for heat loss, stay comfortable in summer and carry its own weight without visible steel.

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Highly glazed rear extension with full-height sliding glass doors, a glass box corner and a walk-on rooflight

What makes a glass extension work?

Orientation and shading first. A south or west-facing glass wall needs solar-control glass, a deep roof overhang or a brise-soleil, or the room will overheat in July. North-facing glass gives soft, even light and is easier to keep comfortable.

Then the energy rules. In England the simple route limits new glazing to 25% of the extension floor area plus any openings removed; a heavily glazed design proves compliance with a whole-house calculation and compensating insulation elsewhere. We run that calculation before the design is fixed.

How much does a glass rear extension cost?

Allow £2,800–£4,800 per square metre including VAT. A 3 m × 4 m glazed extension typically comes to £33,500–£57,500, and 4 m × 6 m to £67,000–£115,000. Frameless glass, structural glass beams and motorised rooflights sit at the top of the range.

Planning and Building Regulations

In England a single-storey rear extension can often be built under permitted development: up to 3 metres beyond the original rear wall of an attached house, or 4 metres on a detached house, no higher than 4 metres, with eaves no higher than 3 metres if any part is within 2 metres of a boundary.

New fabric must meet Approved Document L (2021 edition incorporating 2023 amendments): walls 0.18, roof 0.15 and floor 0.18 W/m²K, windows 1.4 W/m²K or Band B. Under the simple route, new windows, doors and rooflights should not exceed 25% of the extension floor area plus any openings removed; more glass is possible with a whole-house calculation.

Different rules in Wales, Scotland and Northern Ireland. The limits above are for England. Check your own house with our depth checker or read how far you can extend without planning.

What is included?

Overheating and orientation check
Energy calculation for the glazing area
Slim-frame sliding or bifold doors
Structural glass or glass-box corners
Walk-on or fixed rooflights
Solar-control and safety glass
Concealed drainage and level thresholds
Building control sign-off

Guide prices by region

Per square metre of new floor, including VAT, standard specification.

Region	Per m² inc VAT
London	£3,640–£6,240
South East England	£3,140–£5,380
East of England	£2,970–£5,090
South West England	£2,910–£4,990
West Midlands	£2,800–£4,800
East Midlands	£2,720–£4,660
North West England	£2,630–£4,510
Yorkshire and the Humber	£2,600–£4,460
North East England	£2,520–£4,320
Wales	£2,600–£4,460
Scotland	£2,660–£4,560
Northern Ireland	£2,440–£4,180

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FAQ

Glazed rear extension questions

Will a glass extension be too hot?

Not if it is designed for its orientation, with solar-control glass, shading and ventilation.

Is there a limit on how much glass I can have?

In England the simple rule is 25% of the floor area plus openings removed; more is allowed with a calculation showing the house still meets Part L.

Sliding or bifold doors?

Sliders give bigger panes and slimmer sightlines; bifolds open the whole width. Our guide compares them.

Can I have a level threshold?

Yes, with a drainage channel outside the doors and the slab set to the patio level.

Is a glass box a conservatory?

No. It is built to Building Regulations as part of the house, with insulated floor and roof, so it is usable all year.

Does glass need planning permission?

The same rules apply as any rear extension; glass does not change the depth or height limits.

Where