Part L and a material change of use: what actually applies

Part L and a material change of use are a different test from Part L and a new building, and mixing them up is how conversion projects end up over-specified or non-compliant. Where a building undergoes a material change of use under the Building Regulations, the requirement is not to meet the new-build target emission rate. It is to bring the building up to a standard set by the change of use provisions: renovating or upgrading thermal elements where work is being done to them, meeting standards for controlled fittings and services, and satisfying the specific requirements that apply to the new use. That is a lower and more targeted obligation, and it has to be read alongside whatever the local plan asks for on top.

For a landlord converting offices to flats under Class MA, or a house to an HMO, that distinction is worth real money.

What counts as a material change of use

The Building Regulations define material change of use by reference to specified changes rather than by the planning use class. The list includes a building becoming a dwelling, containing a flat where it previously did not, becoming a hotel or boarding house, becoming an institution, becoming a public building, becoming a shop, and changes to the number of dwellings in a building.

Two points follow. First, the building control definition does not track the Town and Country Planning (Use Classes) Order 1987. A change that needs no planning permission because it stays within Class E can still be a material change of use for building control, and a change permitted under Class MA prior approval is one. Second, whether a material change of use has occurred is a question for the building control body, and the answer is worth getting in writing early.

The planning route and the building control route run in parallel and neither waits for the other. A prior approval under Class MA of the General Permitted Development Order 2015 does not carry building regulations approval with it.

Which Part L requirements are triggered

On a material change of use the relevant obligations in Approved Document L, volume 1 for dwellings and volume 2 for buildings other than dwellings, concern the work being carried out rather than the building as a whole.

The practical reading is that you comply element by element. Replace a roof covering and the roof becomes a renovated thermal element with a standard to meet. Leave a wall untouched and it may still need upgrading if its performance falls below the threshold in the Approved Document and upgrading is feasible, which is a judgement recorded with the building control body.

Upgrading thermal elements and the exemptions

The renovation standard for a thermal element applies where a defined proportion of its surface area is being renovated, and the standard is less demanding than the new-build figure. Where the element is retained untouched, the test is whether it falls below the threshold value and whether upgrading is technically, functionally and economically feasible.

The feasibility test is where most of the argument happens on older buildings. Internal insulation to a solid wall reduces floor area and creates condensation risk at junctions. External insulation changes the appearance, which in a conservation area or on a building of traditional construction may be unacceptable, a conflict examined in external wall insulation in a conservation area.

Historic and traditionally constructed buildings have their own treatment. The Approved Document recognises that special considerations apply to listed buildings, buildings in conservation areas and scheduled monuments, and to buildings of traditional construction with permeable fabric, where the aim is to improve performance without causing moisture problems. That is a route to a reasoned lower standard, not a blanket exemption, and it needs to be argued with evidence.

Controlled fittings and services

These are the straightforward part and the part most often skimped. Any window, rooflight or door provided as part of the work has to meet the standard for controlled fittings. Any new heating system, hot water system, ventilation or fixed lighting has to meet the standard for controlled services, including controls and commissioning.

Where a conversion installs a new heating system, that is also the moment the electrification question arises, and the decision interacts with both the emissions picture and any local policy requirement. The reasoning in all-electric new homes and the gas connection decision applies to conversions with the added constraint that the existing fabric limits what a low temperature system can deliver.

Ventilation deserves its own mention on conversions. Reducing air permeability without providing adequate ventilation is the classic conversion failure, and it produces condensation and mould complaints within the first winter.

Where planning policy asks for more than the Regulations

The Regulations are a floor. Local plan policies routinely require more, and the requirement usually arrives as a percentage carbon reduction, a fabric energy efficiency standard, or a renewable energy contribution.

Two difficulties arise on conversions. The percentage reduction policies are generally written against a new-build baseline, so applying them to a change of use needs a stated baseline and a note of how it was derived. And on-site renewable requirements can be physically impossible on a converted building with a small roof already occupied by plant.

The workable approach is to state the policy, state what the building can achieve, evidence the constraint and propose the best available outcome. That evidence belongs in the Energy Statement submitted with the planning application, and it is a different document from the building control submission even though the calculations overlap.

Evidence for the planning submission

1. Identify the existing building, its construction, its age and its current fabric performance, with the basis for each figure.
2. State whether the proposal is a material change of use for building control purposes, and note the building control body's position if you have it.
3. Set out which thermal elements are being renovated and which retained.
4. For retained elements, state the feasibility assessment and its conclusion.
5. Set out the proposed controlled fittings and services standards.
6. Compare the outcome with the relevant local plan policy requirement and explain any shortfall with evidence.

Where a whole life carbon comparison is also required, the reuse of the existing structure is usually the strongest part of the case, and it is worth quantifying rather than asserting.

Sequencing building control and planning

The two processes are independent, and the risk is that a planning permission commits you to something building control will not accept, or the reverse.

The order that works: establish the building control position on material change of use and on thermal element feasibility before the planning submission is finalised. Then the energy statement can describe a strategy that will survive. Doing it the other way round produces a planning condition requiring a carbon reduction the fabric cannot deliver, and then a section 73 application to fix it.

One honest point. On older buildings the gap between calculated and actual performance is wider than on new build, because assumed U-values for unknown construction carry real uncertainty. Say so, and prefer intrusive investigation of the fabric over a table of default values where the outcome matters.

This is general guidance, not legal advice. Check the position with your local planning authority, your building control body or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.