Glazing in home extensions


One of the most commonly encountered restrictions with home extensions is the limit on windows / glazing in Approved Document Part L1B of the Building Regs.

This dictates that the glazed area of all windows must not exceed 25% of the overall floor area.

Even a fairly small extension with a set of patio doors and roof lights can use up this 25% allowance. However there are ways around this rule – See The Haynes Home Extension Manual.


How do I build a Building Regulations compliant extension?

Householders looking for more contemporary designs with larger areas of glazing may struggle to meet the 25% rule.  So the first thing to note  is  that you can add to the permitted total the area of any old window orglazed door openings that are going to be covered over by the new extension.

And there are other ways to demonstrate compliance with the glazing requirements. One method that can be used requires an energy calculation involving the averaging out of all of the ‘area weighted ‘U’ values’ of the extension and comparing them to the average U value of a notional extension of the same size where the minimum standards of L1B have been met including glazed areas. If the proposed ‘area weighted average U value’ is lower than that of the notional extension then the proposed extension is compliant.

An even more flexible method of achieving compliance is to calculate the CO2 emissions for the building as a whole, including the extension, using SAP 2005. This figure is then compared to a notional extension calculation* and if the CO2 emission rate is lower in the proposed building compared to the notional extension*, the proposed extension complies.

There is a major advantage to including the whole dwelling in this calculation. When improvements need to be made to reduce the CO2 emission rate it would be easy to assume that those additional improvements would be made to the extension itself, but if it is to be highly glazed other substantial improvements to the constructional fabric would be required to achieve the same standard as a notional building.

In many cases, the best method of enhancing the building to meet the building regulations is to improve the existing dwelling as a whole. This could include additional loft insulation, a replacement boiler or even cavity wall insulation.

By using the existing dwelling to enable the extension to meet the required standard, you are making substantial improvements – this is ideal in older dwellings which probably have lower standards of insulation and older heating systems. In the case of a more recently built property, there may be more difficulties in making the building comply and may have to consider the addition of renewable energy sources such as solar panels to change the compliance calculations.

Example project:  Mid terraced house in the home counties

Existing dwellings built approximately 1930s, gas fired back boiler in living room approximately 20 years old. The proposed extension has a floor area of 12.78m2, which has 5.64m2 in windows and 2.62m2 in rooflights. The extension is covering over 2.52m2 of existing windows.

Under the guidance of Approved Document L1B the glazing would be limited to a maximum of 5.715m2, so therefore the extension is over glazed by 2.5m2.

If the extension met the notional requirements of approved document L1B, the dwelling as a whole would have a CO2 emission rate of 4.8 tonnes per year. Therefore the proposed extension and the original dwelling must have a lower CO2 emission rate than the notional building.

If the roof of the existing part of the dwelling has a 100mm layer of insulation added the CO2 emission rate will reduce to
4.72 tonnes per year, which is below the notional building figures and therefore compliant with Part L1B.

In this particular case as an alternative to additional insulation the client, as part of overall building improvements installed a new gas fired condensing combi boiler with new radiators, timers and controls. This has further reduced the measured CO2 emission rate which is now 3.12 tonnes per year. This is well below the notional figure of 4.8 tonnes per year.

This extended building, although highly glazed, will now emit 35% less CO2 than a building that would have met the apparent requirements of the Building Regulations. It is critical that the whole energy calculation is completed to assess the level of improvement, as the emission rates can vary significantly from dwelling to dwelling due to age and orientation and overall efficiency of the original dwelling.

This calculation demonstrates that by applying Approved document L1b correctly, accepting SAP calculations from accredited energy assessors using Elmhurst Energy software as a method of demonstrating compliance, a far cleaner dwelling from a CO2 point of view can be achieved with ease.

It should also be taken into account that a more energy efficient dwelling is usually cheaper to run and provides a great deal of design flexibility so that the home owner can have the “Grand Design” they desire.


*  Notional Extension: An extension to a dwelling meeting the minimum standards of approved document Part L1B.



QUESTION:    Which parts of the Building Regs apply when you fit new windows ?



ANSWER:      Quite a few:-


A: Structure

B: Fire Safety

C: Site Preparation and resistance to moisture

E: Resistance to the passage of sound (including the 2004 Amendment

F: Ventilation

K: Protection from falling, collision and impact

L1A Conservation of fuel and power (New Dwellings)

L1B Conservation of fuel and power (Existing Buildings)

M: Access to and use of buildings

N: Glazing – safety in relation to impact, opening and cleaning


PLUS  HSE Information Sheets:

Construction Sheet No.2 (revised): Safe use of ladders

Construction Sheet No. 10 (revised): Tower Scaffolds