Replacing windows in a flat or maisonette

Replacing windows in a flat or maisonette is not permitted development, because Part 1 of Schedule 2 to the GPDO 2015 applies only to a dwellinghouse and the Order excludes a building containing one or more flats and a flat within such a building. That does not mean every window replacement needs planning permission. It means the only question is whether the work amounts to development at all, and that turns on section 55 of the Town and Country Planning Act 1990.

Under section 55(2)(a), works for the maintenance, improvement or other alteration of a building that affect only the interior, or that do not materially affect its external appearance, are not development. Like-for-like replacement in the same material, profile and opening pattern usually falls outside. Timber sashes swapped for white plastic casements do not.

Why flats have no householder rights

The distinction trips up landlords who have done extensions on a house and assume the same rules travel.

Part 1 confers rights on the "dwellinghouse", and the GPDO's interpretation provisions make clear that the term does not include a building containing one or more flats, or a flat contained in such a building. So Classes A to H, covering extensions, roof alterations, porches, outbuildings and hard surfaces, are all unavailable. The same applies to a maisonette, which is a flat arranged over more than one storey.

One useful consequence: an Article 4 direction removing householder permitted development rights has no effect on a flat, because there are no rights to remove. That is not a loophole, since the section 55 test applies regardless of designation, but it does mean the answer in a conservation area is the same as the answer outside one. What changes is how an officer judges "materially affect".

Where a flat is in a listed building, listed building consent is a separate requirement and the test is different again. Consent is needed for works affecting the character of the building as one of special architectural or historic interest, and window replacement in a listed building almost always does.

When replacement becomes development

The threshold is a material effect on external appearance, judged as a matter of fact and degree. Six variables do the work.

Taken individually, one change may be immaterial. Taken together, four changes on a principal elevation in a uniform terrace will be material. Officers judge it from the street, and photographs of the existing condition alongside a window schedule and a 1:5 section through the proposed frame are what settle the point.

Rear elevations not visible from any public place are often treated more flexibly, though "not visible from the highway" is not the statutory test and a rear elevation facing a communal garden overlooked by thirty flats is still an external appearance.

Conservation areas and listed buildings

In a conservation area, section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990 requires special attention to preserving or enhancing character and appearance. That raises the bar on what counts as material, in practice, because the officer is assessing a change against a designated character.

A workable approach for an older block:

1. Survey what is original. Photograph every window on the elevation and record the profile, the horn detail, the meeting rail position and the glazing pattern.
2. Decide between repair and replacement on the evidence. Repair with slim-profile double glazing or secondary glazing is often accepted where wholesale replacement is not.
3. Where replacement is proposed, match in timber with the same opening method and sightlines, and put the sections on the drawings.
4. Treat the whole elevation as one scheme, rather than replacing windows flat by flat as leases turn over.
5. Submit a Heritage Statement setting out significance, the effect of the change and why the proposal is the least harmful option available.

That fifth step is the one leaseholders skip and officers refuse without. Our note on removing a chimney stack in a conservation area deals with the same assessment applied to a different element.

Lease, freeholder and service charge issues

Planning is often the easier half of the problem. The lease decides who owns the window and who may change it.

Read the demise first. In many leases the windows, or the glass and frames, are demised to the leaseholder while the structure remains the landlord's. In others the whole window is retained by the landlord as part of the structure, with repair recoverable through the service charge. The alterations covenant then governs: most leases prohibit alterations to the exterior without the landlord's consent, often with consent not to be unreasonably withheld, and the formal route is a licence to alter.

Two further points for landlords and management companies. Where the landlord carries out replacement across a block and recovers the cost through the service charge, the consultation requirements for qualifying works under section 20 of the Landlord and Tenant Act 1985 apply above the statutory threshold, and failure to consult properly limits recovery. And a planning permission does not override a lease covenant, so a consent obtained without the freeholder's licence is of limited use.

Building regulations, escape and fire

Replacement windows are controlled work under the Building Regulations 2010 whether or not planning permission is needed, and three Approved Documents bite.

Part L sets a minimum thermal performance for replacement windows in an existing dwelling, which in practice rules out single glazing except where a conservation exemption applies. Part F requires ventilation provision to be maintained or improved, which is why trickle vents now appear in replacement window specifications where none existed before. Part B governs means of escape: where a habitable room above ground floor relies on an escape window, the replacement has to keep an openable area and clear opening dimensions that allow escape, and changing a side-hung casement for a top-hung unit can defeat that.

Notification is either through a competent person scheme registration or through a building regulations application to the local authority or an approved inspector. Keep the certificate, because a buyer's solicitor will ask for it.

Where the block is a higher-risk building, broadly one of at least 18 m or seven storeys with two or more residential units, the Building Safety Act 2022 regime applies and building work may require approval from the Building Safety Regulator before it starts. Check the position for the specific building rather than assuming, because the consequences of starting without approval are serious.

An illustrative example

This example is illustrative. A first-floor flat in a 1900s conservation area terrace has original timber vertical sliding sashes with horns and a one-over-one pattern. The leaseholder proposes uPVC top-hung casements in white. The officer's pre-application response is that the change to material, opening method and sightline would materially affect external appearance on a principal elevation within a conservation area, so permission is required and would be resisted. The revised scheme is timber sashes, double glazed in a slim unit, matching the horn profile and the meeting rail height, with the 1:5 sections submitted. The rear windows, facing a private yard, are allowed as aluminium in a dark finish. Permission is granted, and the freeholder grants a licence to alter conditional on the elevation being treated consistently as other leaseholders replace theirs.

What to submit

An application for window replacement in a flat is a small application that gets refused for thin drawings more than for policy reasons.

Include existing and proposed elevations at a consistent scale, a window schedule keyed to the elevations, a 1:5 or 1:20 horizontal and vertical section through a typical frame showing sightline and reveal depth, a specification naming material, finish, glazing and opening method, and dated photographs of the existing windows. A Design and Access Statement is worth providing even where it is not required, because it is the place to explain the matching, and a short Planning Statement carries the policy case.

Check the lease before you check the planning position. Enter your postcode on PlanScope to see which reports an application on your building is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority, your freeholder and a qualified professional before relying on it, and remember that a lawful development certificate is the only formal way to confirm that works are lawful without permission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.