Windows for extensions have to do more than let in natural light. They need to pass the regulations, respect planning rules and look right next to the windows on the original house. In London, with its terraces, protected streets and tight boundaries, that last part matters more than almost anywhere.
We make bespoke timber windows and doors, and an extension project brings up the same questions every time, whether it’s a kitchen extension, a side return extension or a loft roof extension. Do I need planning permission? Will Building Control accept the windows? Should they match the old ones? This guide answers them, so you can brief your architect, design team and builder with confidence.
Factory finished
Double glazed as standard
U-value 1.4 W/m²K
Made to measure
Around 20 working days
In this guide
What You’ll Discover in This Article
- The planning rules that affect windows in a rear or side extension
- What Part L asks of the glazing
- How much glass an extension can have, and where glass extensions fit in
- How to match extension windows to Victorian, Edwardian and 1930s houses
- How we supply the windows, and who fits them
01 · Planning
Planning Rules for Extension Windows
Many single-storey extensions are built without a full planning application. The windows still have to meet a few conditions, and in London those conditions often get stricter.
Size Limits for a Rear Extension
Under permitted development, a single-storey rear extension can usually project up to 4 metres from the original rear wall of a detached house, or 3 metres for any other house, and can’t be more than 4 metres high. With prior approval from the council, those depths rise to 8 and 6 metres. The Planning Portal sets out the full conditions.
Obscure Glass on Side Elevations
Any upper-floor window in a side elevation must be obscure-glazed and non-opening, unless the opening part is more than 1.7 metres above the floor. That rule catches many London loft dormers and two-storey additions, where windows face a neighbour a few metres away.
Materials Similar in Appearance
The materials of an extension should also be similar in appearance to the existing house. If the house has timber windows, timber is the safest route, especially on a street-facing side.
Article 4 Directions
Much of inner London sits in a conservation area, and many boroughs use Article 4 Directions to remove householder rights. In those places, even rear windows may need consent, and the council will expect windows that suit the house. Check with your borough’s planning team before you order.
02 · Regulations
Part L, Glass Limits and Safety
Planning decides how it looks. The regulations decide how it performs. Your builder or architect will deal with Building Control, but it helps to know what the windows have to achieve.
U-Values
Approved Document L sets a limiting U-value of 1.4 W/m²K for new windows in an extension to an existing home. Our energy-efficient timber windows reach 1.4 W/m²K double glazed, with low-E glass, argon and warm-edge spacers, and triple glazing is available where you want high-performance glazing.
The 25% Glazing Rule
As a guide, the total area of windows, roof windows and doors in an extension shouldn’t exceed 25% of its floor area, plus the area of any windows or doors it covers up. If the glazing in an extension exceeds that, your designer will need to show the design still performs as well overall, usually with better U-values or an energy calculation. That’s why the number of windows, and their size, should be settled early.
Ventilation and Escape
New habitable rooms need background ventilation, so trickle vents are often specified. A room on an upper floor, such as a loft conversion, may also need an escape window large enough to climb through. Building Control will confirm what’s required.
Safety Glass
Glass in doors, side panels and low windows near floor level must be safety glass. Toughened or laminated glass is used in these areas, and laminated glass can be supplied on request.
03 · Glass
Glass Extensions and Glazed Extensions
Many London projects are now built around large areas of glass panels. It’s worth knowing what’s possible, and which parts come from which supplier.
Glass Extension Ideas
A modern glass extension might combine floor-to-ceiling glass, a glass roof and wide doors to the garden, creating a light-filled glass room. A glazed extension can also be simpler: a run of large timber windows, French doors and a fixed light above the worktop in an open plan kitchen. Both maximise natural light; the second is easier to keep within the 25% guide.
Conservatory or Extension?
A conservatory is treated differently. If it’s at ground level, under 30 square metres and separated from the house by external-quality doors, it’s usually exempt from Building Regulations. Knock through to the kitchen and the new space has to meet the same standards as any other room. A garden room is different again: it’s a separate building with its own rules.
Glass Roofs and Lanterns
We don’t make roof glazing. A roof lantern, flat glass rooflights or a fully glazed roof come from a specialist supplier. Plan them alongside the windows, so the glass specification, frame colours and sightlines work together.
Specialist Glazing
A glass box or frameless structural glass wall needs a structural glazing specialist and an engineer. Many homeowners compare them with framed glass in timber, which costs less, is easier to get through Building Control and suits a period home better.
Timber vs Aluminium
Aluminium windows are slim and strong, which is why they’re common in contemporary glass designs and the modern aluminium glass extension. They need to be thermally broken to perform well. Timber gives a warmer look inside, can be painted any colour, and sits more naturally against brick on an older house.
04 · Timber options
Timber Windows, Doors and Fixed Lights
The best design depends on the house, the room and which way it faces. These are the usual timber options.
Flush Casements
Flush casements sit level with the frame when closed, giving a clean line that still feels traditional. They suit both period and contemporary designs, and our flush casement windows come in single, double and triple versions.
French Doors and Wide Openings
French doors are the classic timber choice for opening a kitchen onto the garden, and they can be paired with fixed lights to widen the opening. Wider openings are often filled with sliding doors or bifold doors, while French doors keep a traditional look.
Fixed Lights
Fixed lights don’t open, so they can be larger than opening windows. They’re useful for a high-level window above a worktop, or a tall panel of large glass next to French doors.
Sashes on Period Houses
On a Victorian or Edwardian house, sash windows can repeat the style of the main house, especially where it’s visible from the street.
05 · Matching
Matching Extension Windows to the House
In London, a new extension usually sits next to an older house. Choosing the right windows is what makes it look like a natural addition to your home.
Victorian Terraces
Victorian terraces typically have tall windows with slim glazing bars, often two over two or one over one. An extension for a terraced house can repeat those proportions, or use flush casements with a similar bar pattern for a simpler look.
Edwardian Houses
Edwardian houses often have lighter, more decorative windows, with small panes in the upper sash or casement. Copying that detail ties old and new together.
1930s Semis
Many 1930s semis in outer London have casements with horizontal bars, and some have curved bays. Flush casements with a horizontal bar, painted to match, work best on a 1930s house extension.
Contrast or Match?
Some architects build a deliberate contrast into the extension design, with modern glazed walls set against a period house. Timber works for both: a dark RAL colour and large panes give a contemporary look, while a heritage colour and glazing bars blend in.
06 · London
London Considerations
A few things come up more often in London than anywhere else.
Noise
Busy roads, railways and flight paths make noise a real issue in parts of London. Acoustic glass can be specified to reduce noise, and triple glazing helps too.
Access and Delivery
Many London houses have no side access, so windows for your extension may have to come through the house. Check the largest item will fit through the front door and hallway, and tell us about any access limits when you order. Our page on timber windows for London homes has more on supply to London.
Security
Ground-floor doors and windows at the back of a house are often less overlooked. Ask for good locks on every opening window, and laminated glass where it makes sense.
07 · Ordering
Ordering and Delivery
Windows are usually ordered once the structure is far enough along to measure the openings accurately.
When to Measure
Measure from the finished openings, not the drawings, wherever you can. Your builder or architect can confirm the final sizes before you order.
Lead Time
Our windows are made to measure in around 20 working days, factory finished in any RAL colour or a Farrow & Ball match. Plan the order so the windows arrive when the extension works are ready for them.
Fitting
We’re supply only: we make, finish and deliver the windows, and your builder, joiner or a FENSA-registered installer fits them. The quality of the windows only shows if they’re fitted well, so agree early who’ll install and seal them.
Frequently Asked Questions
Do extension windows need planning permission?
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Not always. Many single-storey extensions don’t need an application, but upper-floor windows on a side elevation must be obscure-glazed and non-opening below 1.7 metres, and Article 4 Directions can remove those rights.
What U-value do extension windows need?
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Approved Document L sets a limiting U-value of 1.4 W/m²K for windows in an extension. Our double-glazed timber windows reach 1.4 W/m²K.
How much glass can an extension have?
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As a guide, windows, rooflights and doors shouldn’t exceed 25% of the floor area, plus any openings it covers. More is possible with an energy calculation.
Do you make roof lights or glass roofs?
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No. We make timber windows, fixed lights and French doors. Those come from a specialist supplier.
Do you fit the windows?
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No. We manufacture and deliver across London and the UK. Your builder, joiner or a FENSA-registered installer fits them.
Summary
Conclusion: Plan the Windows Early
Windows for a home extension are easier when they’re planned early: check the planning rules, confirm the U-values with Building Control, and decide whether to match or contrast with the house. Do that, and the windows will make the new living space feel like part of the home.
Ask us for a quote with your window schedule or opening sizes.
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