Designing for Change of Use to a Dwelling – Guide to Compliance
Created May 2011, Webpage last updated: 21 June 2022
About Approved Document L1B and change of use of a building to a dwelling
Approved Document L1B provides guidance on complying with Requirement L1 dealing with the conservation of fuel and power. It is one of four Part L Approved Documents and deals specifically with work on existing dwellings. This guide is to help you to understand Part L1B of the Building Regulations as applicable to schemes involving the change of use of a building to a dwelling. This includes garage and loft conversions. It has not been possible to deal with every issue in full detail and you can contact your local authority Building Control office for further advice on any aspect. Separate guides deal with extensions to dwellings, loft conversions and garage conversions.
Approved Document L1B outlines the energy performance standards required for ‘Thermal Elements’ that form part of the conversion of a building to a dwelling. These are walls floors or roofs that separate the heated space from the external environment or from unheated spaces such as garages. The three types of thermal elements are ‘new / replacement’, ‘renovated’ and retained’ and all may be relevant to a change of use.
The document also gives guidance on the specification of ‘Controlled Fittings’ such as windows, doors and roof lights and ‘Controlled Services’ such as lighting, heating, mechanical ventilation and air conditioning.
Thermal Elements
New or Replacement Thermal Elements
Where a scheme involves the construction of new or replacement walls, roofs or floors, the U-values should meet the standards in Table 1 below
Table 1 – U–Values for New or Replacement Elements
U-Value (W/m² °K)
| Wall* | 0.18 |
| Pitched Roof – insulation at ceiling level | 0.15 |
| Pitched roof – insulation at rafter level | 0.15 |
| Flat Roof or Roof with integral insulation | 0.15 |
| Floors ** | 0.15 |
* Area weighted average values
** Lesser standards accepted where matching floor levels present difficulties
Windows, Rooflights and Doors
Where the work involves the fitting of new windows, doors or rooflights in the external walls, they should meet the standards in Table 2 below.
Any existing unit that does not achieve a U-value less than 3.3W/m² °K should be replaced with units meeting the standards in Table 2 below. The values given are the required area-weighted averages. All units must be draft proofed. (See Historic Buildings below)
Table 2 – Energy Ratings and U-Values for Windows, Doors and Rooflights
| Fitting | Standard |
| Windows, Roof Windows and Rooflights | Maximum 1.4 W/m² °K or Window energy rating Band C or better |
| External doors | Maximum 1.4W/m² °K or door energy rating Band C or better |
| External Doors >60% Glazing | Maximum 1.4W/m² °K or door energy rating Band B or better |
As for new buildings, there is an upper allowable limit on the area of doors, windows and rooflights of 25% of the floor area of the building. If this is exceeded then you will either need to reduce the area or prove that the proposed building will not create more carbon dioxide emissions than a notional equivalent conversion that is compliant with the 25% rule. This is usually done by a specialised energy assessor using accredited SAP 2009 software. The performance of other elements may need to be enhanced to compensate for the added heat loss through the openings.
Renovated Thermal Elements
L1B requires the upgrading of performance in thermal elements where the developer proposes to renovate more than 25% of the surface area. This would typically be work to the inner or outer lining of an external wall, re-covering of a roof or renovation of a ceiling. Re-decking a floor would also be applicable.
The required standards are shown in column (b) of Table 3 below. An exception to the need to upgrade performance is where the upgrade is not technically or economically feasible. The document describes a method of assessing economic feasibility by calculating the ‘simple payback period’ or the amount of time it would take for energy cost savings to exceed the basic cost of upgrading the thermal performance. The section below dealing with historic buildings gives other exceptions.
Retained Thermal Elements
These are thermal elements that exist in the building subject to a change of use and that the designer intends to retain. L1B requires that such walls, roofs and floors be upgraded if their U-value fails to meet the ‘threshold value’ in column (a) of Table 5 below. The target values for improvement are listed in column (b) however the exceptions rules on feasibility apply equally to this type of thermal element. If the upgrade is not technically or economically feasible then the designer should try to achieve the best standard that is possible within the criteria in the previous
paragraph.
Table 3 – Upgrading U-Values of Renovated and Retained Elements
|
|
(a) Threshold Value |
(b) Improved Value |
| Cavity Wall (suitable for the installation of cavity insulation) |
0.70 W/m² °K |
0.55 W/m² °K |
| Other Wall |
0.70 W/m² °K |
0.30 W/m² °K |
| Pitched Roof – insulation at ceiling level |
0.35 W/m² °K |
0.15 W/m² °K |
| Pitched roof – insulation between rafters |
0.35 W/m² °K |
0.15 W/m² °K |
| Flat Roof or Roof with integral insulation |
0.35 W/m² °K |
0.15 W/m² °K |
| Floors* |
0.70 W/m² °K |
0.25 W/m² °K |
* Lesser standards accepted where matching floor levels present difficulties
Controlled Services
Heating and Hot water
The new rules aim to ensure that new systems meet a minimum standard of energy efficiency. The designer can establish the adequacy of a system by referring to the ‘Domestic Services Compliance Guide’ published by NBS. This lengthy and highly detailed document gives standards for many types of heating systems, fuel and controls. A typical specification for a natural gas-fuelled system would be a condensing boiler with a SEDBUK (efficiency) rating of at least 88% (i.e. condensing type) linked to a fully pumped system with boiler interlock (switches off when no demand for heating) and zone, timing and temperature controls. There are different requirements for dwellings with floor areas over 150m².
L1B requires that every new system is commissioned by a person competent to do so and that the owner is provided with sufficient written guidance to enable him to operate the system efficiently.
Lighting
Light fittings also need to accord with the Domestic Building Services Compliance Guide. This means that at least three out of four fittings should be low energy type i.e. have a luminous efficacy of not less than 45 lumens per circuit-watt and total output greater than 400 lamp lumens. Light fittings supplied by less than 5 watts can be excluded from the overall count. In recognising that low-energy type lamps are now dominating the market A.D. L1B states that standard fittings fitted with low-energy lamps are acceptable.
External lighting must either meet the above efficiency standard with fittings being both manually switched and automatically controlled to switch off when daylight is sufficient, or must consist of fittings that have movement and daylight sensors and lamp capacity not exceeding 100 watts.
Other Controlled Services
L1B places control on the efficiency of mechanical ventilation systems by referring to the Energy Saving Trust’s design guide GPG268 – Energy Efficient Ventilation in Dwellings. Fixed air conditioning systems are required to have an energy efficiency rating of Class C.
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 and the Accredited Construction Details produced by CLG suggest suitable details for achieving this with typical methods of construction. These are available for viewing along with Approved Document L1B in the Planning Portal.
Historic Building Conversions
There is a provision in the Approved Document to take account of the building’s architectural or historic importance in applying the requirements and for listed buildings, this may be mandatory. Whilst designers should strive to achieve the best possible energy efficiency, they must ensure that measures are sympathetic to the existing fabric and appearance. Building Control is able to organise joint consultations with Planning Conservation Officers to discuss and agree on such issues.
Energy Performance Certificates
In forming a new dwelling, a developer has the responsibility of obtaining an Energy Performance Certificate for the building from an energy assessor registered with a government-accredited scheme. This can be produced in conjunction with an ‘SAP’ emissions calculation and is similar to the energy performance labels found on some
electrical goods giving a rating on a scale of A to G. It includes information on how an owner can improve the efficiency of their property. Building Control needs written confirmation that the EPC has been passed to the owner before issuing a completion certificate.
In Conclusion
We hope that this guide will help you to better understand PartL1B of the Building Regulations as applicable to the design of conversions to dwellings. It has not been possible to deal with every issue in full detail and you are very welcome to contact your local authority for further advice on any aspect.
U-Values prior to 15 June 2022
If you need to reference the old U-Values, you can download our previous guide below.
Should I use the old or new U-Values?
Based on regulation updates to new Part L which came into force on 15 June 2022, the following applies:
- Works which commenced and had an application deposited before the 15 June 2022 are allowed to use the previous U-Values
- Works which have had an application deposited before 15 June 2022 and have NOT yet commenced can only use the old U-Values if work commences prior to 15 June 2023
- Works commencing after 15 June 2023 MUST use new U-Values
- Works deposited after 15 June 2022 MUST use new U-Values.
