Designing Domestic Extensions Guide to Compliance – last updated: Aug 2024
Approved Document L1 2021 – Conservation of Fuel and Power
A Guide to Compliance When Designing Domestic Extensions
Approved Document L1B provides guidance on complying with the requirements of Regulation L1 dealing with the conservation of fuel and power. It is one of four Part L Approved Documents and deals specifically with work in existing dwellings. This guide clarifies the document’s contents in relation to the design of extensions to dwelling houses.
A separate guide is available dealing with schemes involving a change of use to form a dwelling. See guide ‘Change of Use to Form a Dwelling’.
Scope
This document outlines the energy performance standards required for ‘Thermal Elements’. These are walls, floors or roofs that separate the heated space from the external environment or from unheated spaces or such as garages. The four types of thermal elements are ‘new’, ‘replacement’, ‘renovated’ and retained’ although it is mainly new elements that are relevant to this particular guide.
This document also gives guidance on the specification of ‘Controlled Fittings’ such as windows, doors and rooflights and ‘Controlled Services’ such as lighting, heating, mechanical ventilation and air conditioning. Section Construction Standards, advises on the need for the builder to provide the house owner with simple instructions on how to use heating and hot water systems and associated controls in a way that achieves maximum energy efficiency.
Walls, Roofs and Floors
Table 1 – U–Values for New Elements
The table shows the maximum thermal transmittance that is acceptable through various elements of a domestic extension based on U-values.
| Wall | 0.18 U-Value (W/m² oK) |
| Pitched or Flat Roof | 0.15 |
| Floors | 0.18 |
The U-value of a floor is calculated on the basis of the ratio between its perimeter and plan area, and it is acceptable to take this as that of the enlarged dwelling. To help designers meet the above standards, Hertfordshire Building Control published a guidance document ‘U-Values of Elements’ https://www.hertfordshirebc.co.uk/guidance-note/u-values-of-elements/ This suggests a variety of compliant specifications using commonly available insulation materials. Building Control will also consider other solutions subject to performance being calculated using the conventions set out in BR443 Conventions for U-Value Calculations
Windows, Rooflights and Doors
These should incorporate draft proofing and meet the performance shown in the table below. The U- values are calculated for the whole unit i.e. the combined performance of glazing and frame. Insulating cavity closures should be incorporated around openings in this type of wall.
We can sometimes accept lower standards of performance in buildings where aesthetic appearance is critical such as buildings of architectural or historic importance
Table 2 – Energy Ratings and U-Values for Windows, Doors and Rooflights
|
Windows |
Maximum 1.4 W/m² oK or
window energy rating band B minimum |
| Rooflights | Maximum 2.2 W/m² oK |
| Doors with > 60% of internal face glazed | Maximum 1.4 W/m² oK or doorset energy rating band C minimum |
| Other doors | Maximum 1.4 W/m² oK or doorset energy rating band B minimum |
Limits on the Total Area of Openings
The area of openings (windows, doors and rooflights) to an extension should not exceed 25% of the floor area of the extension added to the area of windows which no longer exist or are no longer exposed due to the extension. If a designer can’t meet that target then the optional approaches below offer more flexibility.
Optional Approaches
As an alternative to the thermal performance targets in the above tables and the fixed limits on the total area of openings in the preceding paragraph, a designer can use one of the following optional approaches to give greater flexibility. Note that Approved Document C suggests absolute upper U –value limits of 0.7 W/m² oK for walls and floors and 0.35 W/m² oK for roofs. This is to deal with the health risk posed by condensation.
1 – Area Weighted U-Value
A designer has the option to show compliance by carrying out a calculation demonstrating that the ‘Area Weighted U-Value’ of all the thermal elements and exposed openings is no greater than that of an extension of the same size and shape that complies with the elemental U-value standards and does not exceed the 25% limit on area of openings.
The area-weighted U-value is given by the following expression
[(U1 × A1 ) + (U2 × A2 ) + (U3 × A3 ) + …]
(A1 + A2 + A3 + …)
U1 = the U-value of element type 1, A1 = the area of element type 1 and so on.
2 – Whole Dwelling Standard Assessment Procedure (SAP)
A designer can exploit even more design flexibility by carrying out a SAP (version 10) energy rating calculation for the whole extended dwelling. Using this method a comparison is made with the expected carbon emissions and energy performance of a ‘notional’ dwelling with an equivalent (same size and shape) extension that complies with the fixed limits set out above. This method gives a designer the potential for ‘trade-off’ by improving the performance of the existing house. The upgrades must at least meet the standards in table 3 below. We ask that all SAP calculations are carried out by an accredited energy assessor.
Table 3 – Minimum Standards for Upgrading Existing Thermal Elements
U-Value (W/m² oK)
| Wall – cavity insulation | 0.55 |
| Wall – external or internal insulation | 0.30 |
| Floor | 0.25 |
| Flat or pitched roof – insulation at ceiling level | 0.16 |
Extensions that are Exempt from Building Regulations
Under schedule 2 class 7 of The Building Regulations, some extensions are exempt if they have an internal floor area of less than 30 square metres and consist of a conservatory, porch, carport (open on at least 2 sides) or covered way. The accepted definition of ‘conservatory’ for building control purposes is a ground level extension that has not less than three quarters of its roof area and one half its exposed wall area made from translucent material. In the interests of fire safety, Herts Building Control accepts that walls that are within 1 metre of the facing boundary can be excluded from this calculation. To be exempt from control, conservatories and porches have to be’ thermally separate’ (see below) from the rest of the house and any glazing in critical zones must be toughened or laminated safety glass.
Please note that full exemption does not apply if the heating system of the house is extended into the new room or if it has its own fixed heating appliance. In such cases we require an application to ensure that the design shows compliance with the Part L provisions for thermal efficiency. Exemption from the other requirements stands.
You should also note that regulatory requirements relating to electrical wiring still apply irrespective of the conservatory’s exempt status. Contact Building Control or see our guidance leaflet on Part P – Electrical Safety if you are unsure whether this is relevant.
Non-exempt Conservatories and Highly Glazed Extensions
It is common for conservatories that are not exempted from control to exceed the 25% of floor area limit placed on total area of openings. Approved Document L1 allows that such designs can be acceptable if all the following criteria are followed:
- The extension must be ‘thermally separated’ from the existing by walls, doors and windows giving a level of thermal insulation equal to or better than exposed elements to the existing dwelling. Openings must be draft-proofed.
- The heating system to the room must have independent temperature and on/off controls and any heating appliance must be reasonably efficient i.e. accord with the associated standards set out in the Approved Document.
- U values for wall,roof and floor elements and for doors, windows and rooflights must accord with tables 1 and 2
Controlled Services (Heating and Hot water)
Where schemes include the extension or replacement of an existing system, the rules in section 6 of Approved Document L1 aim to ensure that the new system meets a minimum standard of energy efficiency and is no less ’carbon efficient’ than the original. The designer can establish the adequacy of a new system by referring to the relevant section depending on appliance and fuel type. A typical specification for a replacement natural gas boiler serving radiators or underfloor heating would be a condensing unit achieving 92% efficiency as displayed on the energy rating label that is legally required to be on the packaging.
Every new system must be designed by a competent person on the basis of hot water demand and by using heat loss calculations so that it is not oversized and various industry bodies provide guidance on that process. If you choose to totally replace a heating system then it must be designed to operate at a flow temperature no higher than 55oC . That’s ideal for underfloor heating but necessitates larger radiators than with older systems.
Systems must be commissioned by a person competent to do so and the owner must be provided with sufficient written guidance to enable him to operate a new heating, cooling or ventilation system efficiently. Gas installations must be undertaken by a person who is listed with the ‘Gas Safe’ register.
Controlled Services (Lighting)
Internal light fittings need to be fitted with lamps with a minimum luminous efficacy of 75 light source lumens per circuit-watt. Modern low energy fittings can usually achieve that.
External lighting must also meet the above efficiency standard with fittings being both manually switched and automatically controlled to switch off when daylight is sufficient. If efficiency is less than 75 light source lumens then the system must include movement sensors i.e. PIR.
Other Controlled Services
Approved Document L also covers minimum standards in relation to mechanical ventilation, air conditioning and renewable energy systems (heat pumps and solar panels). Fixed air conditioning systems are required to have a seasonal energy efficiency ratio of at least 4.0 or better and need to be carefully designed to avoid oversizing.
Construction Standards
Irrespective of the quality of materials used, there is a major potential for heat loss and cold bridging caused by poor standards of construction in terms of airtightness and the continuity of insulation. For this reason, it is important for the designer to ensure that the various insulated elements in a building are carefully detailed at junctions. Uncontrolled air leakage can be minimised by specifications calling for appropriate levels of sealing. The Accredited
Construction Details produced by LABC are a useful reference for achieving best practice when using traditional methods of construction. These are available for viewing at. https://www.labc.co.uk/professionals/registration-schemes/registered-construction-details
…In Conclusion
We hope that this guide will help you to better understand PartL1 of the Building Regulations as applicable to the design of your domestic extension. For any queries you have on your project that is submitted with us, do contact us.
