Retrofitting flats: fabric works and consents

Retrofitting a flat runs into a rule most landlords discover late: the householder permitted development rights in Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 apply to dwellinghouses, and a flat is not a dwellinghouse for those purposes. So an external alteration to a flat that amounts to development needs planning permission, even where the identical work on a house next door would not. Add a landlord's licence under the lease, listed building consent where the building is listed, and Building Regulations approval, and a single fabric upgrade can involve four separate permissions running on different timescales.

What needs permission in a block

Start with whether the work is development at all. Section 55 of the Town and Country Planning Act 1990 excludes works that affect only the interior of a building, and works that do not materially affect its external appearance. Internal insulation to an unlisted flat therefore usually needs no planning permission.

External work is the other way round. Replacing windows, adding external wall insulation, installing a flue or a heat pump, changing a roof covering, adding rooflights or altering a balcony all affect the exterior, and with no Part 1 rights available the question becomes whether the change is material.

Part 14 is the exception that matters, because unlike Part 1 it does extend to blocks of flats for some equipment, including air source heat pumps under Class G and solar equipment. The conditions attached to those classes have been amended more than once, so read the current text of the Order rather than a summary, and check the relevant certification standards the class refers to. Array limits on flat roofs are covered in flat roof photovoltaics and permitted development limits.

Window replacement and design policy

Windows are the most common retrofit application and the most commonly refused. The objection is almost never about thermal performance. It is about pattern, reveal depth, frame section and material across an elevation that reads as one composition.

Three things improve the chances. Replace the whole elevation or the whole block in one application rather than one flat at a time, because a consistent scheme is easier to support than a patchwork. Match the original sightlines, opening pattern and reveal depth, and say on the drawings what those dimensions are. And provide large-scale section details at head, jamb, cill and transom, because that is where a conservation officer looks.

Slim double glazing in a timber frame is often the answer in a sensitive setting. Secondary glazing is the answer where the primary windows have to stay, and it performs better than most people assume, particularly for noise.

Internal insulation and moisture risk

Internal wall insulation is attractive because it usually avoids planning permission. It is also the intervention most likely to cause damage, because it moves the dew point into the existing wall and reduces the drying capacity of the construction.

Do the risk assessment before the specification. That means identifying the wall construction, whether it is solid or cavity, whether the external finish is permeable, the condition of rainwater goods and pointing, the presence of embedded timbers such as joist ends and wall plates, and the existing ventilation. Retrofit assessed and delivered under the PAS 2035 framework follows this sequence formally, with a retrofit assessment, a risk path and a coordinator, and following that structure is worth it even where funding does not require it.

Two junctions cause most failures. Joist ends built into an insulated external wall lose the heat that used to keep them dry, and the party wall junction creates a cold bridge where insulation stops. Both need detailing rather than optimism.

Ventilation is not optional. Reducing air leakage without providing controlled ventilation moves moisture from the fabric to the air and produces condensation and mould, so the Part F provision has to be designed alongside the insulation.

Communal plant and riser interventions

Anything that touches the communal system moves the project from one flat to the whole building. Replacing a communal boiler with a heat pump affects flow temperatures, so it usually needs emitter replacement in every flat, and that means access to every flat.

Riser interventions bring their own consents: fire compartmentation at every floor penetration, and on a higher-risk building the building safety regime applies with its own gateways. Where new plant goes on the roof, the planning question is visual and acoustic, and the overheating consequence of higher internal temperatures or restricted openings belongs in an Overheating Risk Assessment. Where mechanical cooling is being considered, the policy expectation is that passive measures come first, as set out in the cooling hierarchy.

Leasehold consents running alongside planning

Planning permission does not give a right to do the work. The lease usually prohibits alterations without the landlord's written consent, and external works almost always affect the structure or the common parts, which are retained by the landlord.

• Check the lease for the alterations covenant and whether consent is subject to reasonableness.
• Apply for a licence for alterations, with the same drawings submitted for planning.
• Expect the landlord to require a specification, evidence of insurance, and payment of its surveyor's and solicitor's costs.
• Where the freeholder intends to recharge major works through the service charge, the statutory consultation requirements for qualifying works apply and take time.
• Where work affects a party wall or involves excavation near a neighbour's structure, the Party Wall etc. Act 1996 procedure runs separately again.

None of these are planning matters, and all of them can stop a programme that has planning permission.

Conservation area and listed building constraints

In a conservation area the authority has a duty under section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990 to pay attention to preserving or enhancing character, and an Article 4 direction may have removed rights that would otherwise apply. For a listed building, listed building consent is needed for works affecting its special interest, and that includes internal work, so internal insulation and secondary glazing both need consent.

The honest position on listed flats is that deep fabric retrofit is constrained, and the useful gains usually come from the roof, the floor, draught-proofing, secondary glazing and services efficiency rather than from the walls.

Sequencing consents and works

Order the work so that the slowest consent is started first.

1. Survey and condition assessment, including moisture and ventilation.
2. Design to a specification detailed enough to submit for both planning and the landlord's licence.
3. Submit planning and listed building consent applications together where both are needed, and the licence application in parallel.
4. Building Regulations application, with the ventilation strategy resolved.
5. Statutory service charge consultation where the freeholder is recharging the works.
6. Procurement and works, with the retrofit sequence set so insulation and ventilation are installed together.

Where the flats are let, the minimum energy efficiency standards for privately rented property set the floor that already applies, and further tightening has been proposed at various points without commencing, so check the position as at the date you are planning the works rather than relying on a summary.

The Energy Statement is the document that explains the fabric and services strategy to the planning authority where an application is needed. PlanScope drafts one structured to the sections authorities expect and flags the site-specific figures it cannot verify rather than filling them in. For specialist technical reports, qualified professional input may be required.

Enter the postcode on PlanScope to see which documents a retrofit application is likely to need.

This is general guidance, not legal advice, so check the position with your local planning authority, your lease and 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.