November 5, 2026

House Extension Glazing Rules: The 25% Allowance and How to Get More Daylight

A House Extension does not have to be mostly glass to feel bright, and the standard glazing allowance is not necessarily the final limit. The key is to balance the amount of glazing, its thermal performance and the direction from which daylight enters.


For a conventional extension, the starting calculation is 25% of the new floor area, plus the area of existing windows and doors covered by the extension. A compensatory heat loss calculation can provide a route to more glazing while keeping the new space open to the rest of the home.


The guidance below focuses on the arrangements described for England and Scotland. Confirm the applicable requirements for your project before finalising the design or ordering windows.



Key Takeaways

  • The basic glazing allowance is 25% of the new floor area, plus existing windows and doors covered by the extension.
  • A compensatory heat loss calculation can support more glass through improved insulation and window performance.
  • Flat-roof glazing needs different thermal consideration from glazing in a pitched roof.
  • Glazing position and orientation can matter as much as its total area.



Table of Contents

Calculate Your House Extension Glazing Allowance First


Before choosing large patio doors or roof windows, establish how much glazing the basic calculation allows. The allowance covers the new glazed openings collectively, rather than giving each wall or roof its own separate budget.


Basic glazing allowance = 25% of the extension floor area + the area of existing windows and doors covered by the extension.


  • When assessing a House Extension, account for all the proposed glazed openings:
  • Windows in external walls.
  • Glazed doors, including patio doors.
  • Roof windows and rooflights.


Worked example: a 20 m² extension

Suppose the new floor area is 20 m² and the extension covers an existing 2 m² door in the rear wall of the house.


That 7 m² must accommodate the whole glazing design. A substantial set of patio doors can use more than half of it, leaving relatively little for wall windows and rooflights.


The common mistake is to treat 25% as either an absolute ceiling or a separate allowance for every surface. It is the starting point for the overall design, with an alternative thermal assessment available where more glass is proposed.

Decide Where Daylight Will Be Most Useful


Once you know the starting allowance, consider where to use it. A bright House Extension depends on the position and orientation of the glass, not simply its total area.


Start with the activities inside the room. A kitchen in the middle of the plan, a reading corner beside the garden and an area used for screen-based work may benefit from different lighting conditions.


South-facing glazing: direct sunlight and warmth

South-facing glass brings direct sunshine into the room. This can feel generous and warming in winter, but a heavily glazed south-facing elevation can become uncomfortably bright and hot in summer.

A design that looks appealing on paper may leave you relying on blinds during the brightest part of the day. Consider the seasonal experience, rather than judging the space only by how much sunlight it receives.


North-facing glazing: softer, more even light

North-facing glazing generally provides diffuse, consistent daylight rather than strong direct sunshine. This quality of light can suit reading, television and laptop use, where glare and screen reflections are unwelcome.


It can also create a calm relationship with a garden. However, concentrating almost all the glazing on one north-facing wall may produce light that is uniform but lacks variation across the room.


Roof windows: daylight deeper into the plan

Light from above can illuminate the floor area more effectively than light entering through a wall. An illustrative comparison is that a 1 m² roof window can light a space more effectively than a 2 m² wall window.


This makes roof glazing worth considering when a House Extension has a kitchen or other important working area away from the external walls. The objective is to bring light where it is needed, not simply to maximise the view out.

How to Exceed the 25% Glazing Allowance


If the basic allowance does not accommodate your design, a compensatory heat loss calculation can assess the thermal performance of the extension as a whole.


The principle is straightforward: better-performing walls, floors and roofs can compensate for additional glazing. More thermally efficient windows can also help.


For an open-plan House Extension, this is the important distinction. You do not necessarily have to choose between a modest amount of glass and a space separated from the existing house by a door.


What is a U-value?

A U-value measures heat loss through a building element. Lower U-values indicate better thermal performance.


When comparing windows, a lower U-value can make a larger glazed area possible within the compensatory assessment. However, the window specification must be considered alongside the rest of the construction.


For example, an illustrative pitched-roof calculation compares roof windows with U-values of 1.3 and 0.83. In an extension with a 20 m² pitched roof and one roof window, the more efficient option increases the possible roof glazing from just over 1 m² to around 1.5 m².


The higher-performing window in that comparison uses triple glazing and costs more. These figures illustrate the effect of improved performance; they are not a universal allowance for every roof of that size.


More insulation is not an automatic approval

Adding insulation or selecting a premium window does not, by itself, establish how much glass your House Extension can have. The proposed construction and glazing need to be assessed together.


Discuss the compensatory calculation while the design is still flexible. That allows you to compare the benefits of improving the building fabric, changing the window specification or redistributing the glazing.

Flat or Pitched Roof: Why the Choice Affects Glazing


Roof form influences both the daylight strategy and the thermal calculation. The basic 25% allowance remains the same, but flat-roof glazing is treated differently from glazing in a conventional pitched roof when assessing additional glass.


Because glazing in a horizontal position loses heat differently from sloping glazing, flat-roof windows need particularly strong thermal performance to support a larger area under the compensatory approach.


A flat-roof window performance and cost example

An illustrative comparison uses VELUX flat-roof windows with U-values of 0.8 and 0.55. For a House Extension with a 20 m² flat roof and one roof window, choosing the lower-U-value model increases the possible glazed area from approximately 1 m² to 1.5 m².


The stated price difference between those two options is around £1,000 plus VAT. This is a comparison between particular window options, not a fixed price for adding more glass to every extension. Obtain a current quotation for the specification you need.


The range includes fixed and opening options, with flat and curved glass. Compare thermal performance as well as appearance and opening arrangements.


Let the existing house inform the roof shape

A pitched roof is not always practical. On a mid-terrace townhouse, for example, a conventional pitched extension roof could obstruct first-floor windows. A flat roof can offer a workable alternative; zinc was used in one such Edinburgh project.


Nor should a flat roof be rejected solely on the assumption that it will leak. Zinc, single-ply membrane and felt have all been used successfully on extension projects. Roof shape and window performance should be considered together, rather than treating either as an isolated choice.

Can a Conservatory Have More Glass Than an Open-Plan Extension?


A thermally isolated conservatory follows a different route from a conventional House Extension that opens directly into the existing home.


The arrangement described allows extensive glazing where the space is:

  • No more than 30 m² in England or 50 m² in Scotland.
  • Thermally separated from the house by a door.


Those size and separation conditions should not be treated as a complete approval checklist. Check the requirements applicable to the proposed structure.


The practical issue is the separation. If your aim is an uninterrupted kitchen and dining space, a door between the old house and the new room may undermine the layout you want. In that case, explore the conventional extension calculation and its compensatory route instead.

Test Different Glazing Layouts Before Committing


A useful way to improve a House Extension is to compare several layouts with the same floor area and the same total glazing allowance. That reveals whether the glass is working hard enough.


Consider a layout with a south-facing entrance beside a driveway, a north-facing rear garden and a flat roof. Three approaches illustrate the trade-offs:

  1. Prioritise the garden elevation. A large north-facing glazed wall creates an expansive view and even light, but leaves less allowance for other openings.
  2. Allocate more glass to the south-facing entrance. Even a relatively modest opening can introduce strong sunlight and warmth, although the rear glazing must reduce to compensate.
  3. Distribute glass between walls and roof. Roof windows bring daylight into the middle of the layout near the kitchen, while smaller wall openings retain views and light from other directions.


The third approach was used in the completed Edinburgh extension behind this example. Its value lies in the balance: daylight reaches the centre of the room rather than entering almost entirely from one side.


Use daylight modelling to assess the experience

Plans show where openings sit, but they can be difficult to interpret as an experience of light. Three-dimensional daylight modelling helps compare how a proposed space will feel at different times and in different seasons.


VELUX Design Studio offers a free daylight visualisation service for homeowners and architects. Its advisers can use project drawings to model the space and discuss relevant product options.


For your House Extension, use those comparisons to ask practical questions:

  • Does daylight reach the kitchen and the middle of the room?
  • Is the light concentrated on one side?
  • Where will direct sunshine fall?
  • Would moving some glazing into the roof improve the balance?
  • How does the proposed glass specification affect the model?

House Extension Glazing Mistakes to Avoid


  • Choosing doors before checking the allowance. Large patio doors can consume much of the available glazing area.
  • Forgetting roof windows in the total. Roof glazing shares the same basic allowance as wall windows and glazed doors.
  • Assuming more glass always gives a better room. Position, orientation and the activities beneath it matter just as much.
  • Comparing flat and pitched roof windows as interchangeable. Their thermal treatment differs in compensatory calculations.
  • Designing only from the outside. An attractive elevation does not establish whether the interior will have useful daylight.
  • Treating example costs as a project quotation. The £1,000 plus VAT comparison relates to particular flat-roof window options.

A Practical Brief for Your House Extension Design


Before settling the glazing specification, work through these decisions with your designer:


  1. Establish the basic allowance. Calculate 25% of the new floor area and add the existing openings the extension covers.
  2. Map how the room will be used. Identify where daylight is most valuable.
  3. Check orientation. Consider direct southern sunshine, diffuse northern light and light from above.
  4. Resolve the roof form. Account for existing upper-floor windows and the performance needed from roof glazing.
  5. Assess alternatives. Compare improved insulation, lower-U-value windows and different glazing positions.
  6. Test the daylight. Use modelling to compare layouts before committing.


The best House Extension design starts with the interior conditions you want to create. Establish the allowance, explore the thermal options and place the glass where it makes the greatest difference.

Frequently Asked Questions

Is 25% the maximum glazing allowed in a House Extension?

No. It is the basic starting allowance, with the area of existing windows and doors covered by the extension added to it. A compensatory heat loss calculation can provide a route to additional glazing.

Do rooflights count towards the glazing allowance?

Yes. Rooflights, wall windows and glazed doors all need to be included when assessing the proposed glazing.

Can triple glazing help an extension have more glass?

Yes, where it improves the window's U-value. Better thermal performance can support more glazing within a compensatory calculation, but the result depends on the complete extension design.

Is a flat roof unsuitable for a bright extension?

No. Flat-roof windows can bring useful daylight into the centre of the layout. Their thermal performance is particularly important when seeking more glazing than the basic allowance permits.

Is north-facing or south-facing glazing better?

Neither is universally better. North-facing glazing provides softer, more consistent light, while south-facing glazing introduces direct sunshine and warmth but can become too bright or hot in summer. Choose according to how the room will be used.

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