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Building Regulations Part L1b: Conservation of Fuel and Power, applies to Existing Dwellings. The regulation is  broad in reach and is intended to make notifiable any significant building modifications that are undertaken.

The Approved Document for Part L1b gives guidance on the way in which compliance with the regulation may be demonstrated. The document is notoriously difficult to navigate.

Part L1b (existing dwellings) is unlike Part L1a (new dwellings) in that different standards for fabric, fittings & services are applied depending on the category of work judged to be undertaken. There is much overlap however, and a single job may qualify under multiple categories.

Part L1b: Existing Dwellings: Some facts

Part L1b is often not well enforced because Building Control Officers have limited budgets even to determine when work that should have been notified has not been; and less to instigate legal action.

However, when dwellings come to be sold, certificates for any notifiable work are required as part of the exchange. This often means that Part L is applied retrospectively to projects where the need for notification was overlooked.

Usually, the regulation need only apply to modified areas. This might mean that a retained supporting wall in a new extension is considered, but an adjacent wall is not. However, it can be an advantage to consider the whole dwelling. To encourage design flexibility, modifications to e.g. the heating system, can be ‘traded-off’ with new elements. This must be demonstrated using SAP methodology.

There area four categories of work, which may or may not overlap:

  • EXTENSION (or Conservatory)

Parallel with this division into categories, there are standards for each element depending on its nature:

  • EXTENSION: Provision of (new) elements: New thermal elements, fittings and services must conform to standards that are similar, and in some places stricter than, those for new dwellings.
  • MATERIAL ALTERATION: Retained elements: Dwellings constructed prior to the early 1980’s typically have fabric U-values worse than threshold values for retained elements. This means that under the categories of Extensions and Material Changes of Use, improvements to existing fabric is typically required.
  • RENOVATION: Renovated elements: Renovation of a thermal element (Improvement to greater than 25% area) also attracts improved standards. Simply re-pointing a single external wall may have implications.
  • MATERIAL CHANGE OF USE: Replaced fittings: Windows and doors are considered as fittings rather than thermal elements. Requirements for glazing will be of interest to anyone carrying out work to existing dwellings.

Advice on the specification of glazing when altering an existing dwelling

  • Single glazed windows have a u-value worse than a threshold of 3.3 W/m2K; if they fall inside the scope of work, then it is most likely that they must be replaced.
  • Guidance in the Approved Document recommends that new glazing in an extension to an existing dwelling should have an area no greater than 25% of the increase in floor area + the area of any glazing removed from walls that are no longer exposed . However, to allow for design flexibility, reductions in heat loss through improvements to other thermal elements can be traded-off for increased glazing areas.
  • Glazing areas may be increased by 15% by considering the elements of the new extension. Furthermore, improvements to the existing dwelling can also be traded-off with the performance of new elements. SAP calculations are required to demonstrate CO2 reductions resulting from house-wide improvements.

Note: Certain buildings, i.e. listed buildings, may be exempt from Building Regulations.