Replacement windows, doors and changing an opening

Replacing windows like for like in an ordinary house needs no planning permission, because the work does not materially affect the external appearance and falls outside the definition of development in section 55 of the Town and Country Planning Act 1990. Cutting a new opening, changing a window into a door or swapping timber sashes for white plastic casements is a different question, and the answer usually turns on Class A of Part 1 of the GPDO 2015, the obscure glazing condition for upper floor side windows, and whether an Article 4 direction or a listing applies.

The condition that surprises people is the one about upper floor side elevation windows, and it applies to far more houses than homeowners expect.

Like for like replacement and the alteration question

A window replaced in the same opening, in the same style, with the same subdivision and the same materials, changes nothing visible and needs nothing from the planning system.

Move away from like for like and the position shifts. Replacing a six over six timber sash with a single pane of glass in a plastic frame alters the appearance of the elevation, so it is development. On an unrestricted house it is normally permitted development under Class A, which covers the improvement or other alteration of a dwellinghouse, subject to the condition that the materials used in any exterior work are of a similar appearance to those used in the existing house.

That materials condition is where plastic windows in a brick and timber street become arguable. Similar appearance is not the same as identical, and councils vary in how strictly they read it, but a wholesale change of frame material across a front elevation is the kind of work that attracts an enforcement enquiry if a neighbour complains.

Flats and maisonettes are outside Part 1 altogether, which applies only to dwellinghouses. In a converted house or a purpose built block, window replacement that changes appearance needs planning permission, and the lease will almost always require the freeholder's consent as well.

Creating a new opening in a side elevation

A new window is an alteration and is covered by Class A on an unrestricted house. What Class A does not do is waive the condition attached to upper floor side windows.

The practical sequence for a new opening is to check four things: whether the house is listed or in a conservation area, whether permitted development rights have been removed by condition or by an Article 4 direction, whether the window is at an upper floor in a side elevation, and whether the opening affects a roof space or eaves where bats may be roosting.

That last point is easy to overlook and carries criminal liability rather than a planning consequence. Works to a roof, a soffit or a gable that disturb a bat roost need the position checked first, and protected species and permitted development rights explains why the Order does not authorise an offence.

Privacy is the reason councils care about side windows, and it is also the reason a new opening overlooking a neighbour's garden attracts complaints even when it is lawful. A high level window, an obscure-glazed panel or a rooflight instead often achieves the same internal result without the argument.

The obscure glazing and opening height condition

Class A carries a condition on upper floor windows in a side elevation. Any window in a wall or roof slope forming a side elevation of the dwellinghouse, at any upper floor, must be obscure-glazed, and must be non-opening unless the parts that can be opened are more than 1.7 metres above the floor of the room in which the window is installed.

Both limbs apply together. Obscure glazing alone does not satisfy it if the window opens at low level, and a high level opening light does not satisfy it if the glass is clear.

Two details matter in practice. The 1.7 metres is measured from the floor of the room, so a room with a raised floor or a split level changes the answer. And obscure glazing means glazing through which a person cannot see, which in practice means a specified privacy level rather than simply textured glass, so put the specification on the drawing.

Where clear glazing or a low opening is wanted, the route is a planning application, assessed on overlooking and neighbour amenity. That is a straightforward application and it is often granted where the outlook is onto a roof, a flank wall or a highway rather than into a garden.

Changing a window to a door

Converting a ground floor window into a door, or a door into a window, is an alteration to the external appearance and is normally permitted development under Class A on an unrestricted house, with the materials condition applying.

Three things frequently take it out of Class A. A door serving a new external area with steps, a platform or a terrace raised above ground level brings its own restrictions, because a raised platform is treated differently from an alteration. A door on a wall that fronts a highway and forms a side elevation is fine as an alteration, but any projecting structure beyond that wall is not. And a new opening that requires a lintel and structural work affecting a party wall engages the Party Wall etc. Act 1996, which is a separate civil process.

Bifold or sliding doors replacing a rear window are the common version of this and are usually straightforward. The complications are structural and thermal rather than planning: a wide opening needs a designed beam, and a large glazed area changes the overheating and thermal calculations for the room.

Conservation areas, Article 4 directions and listed buildings

In a conservation area, Class A rights for windows are generally still available, because the restrictions on designated land bite on cladding and on side and two-storey rear extensions rather than on window alterations. That is the general position, and it is frequently displaced.

Article 4 directions are the reason. Many authorities have removed Part 1 rights over front elevation alterations, including windows and doors, across whole conservation areas, precisely because uncontrolled replacement erodes the character the designation protects. Check the council's Article 4 register and the national planning data service before ordering anything, and check the date of the direction.

Conditions on newer estates do the same job. A condition on the original permission stating that no development within Part 1 shall be carried out without permission means every window change needs an application, and the route through it is an application under section 73 of the 1990 Act, covered in removing a permitted development condition by section 73.

Listed buildings are a different regime. Listed building consent is needed for works affecting the character of the building, and window replacement is one of the works most often carried out without it and most often the subject of enforcement. Historic England's guidance on windows in historic buildings is the starting point, repair is preferred to replacement, and secondary glazing is frequently the accepted answer where thermal performance is the driver. A Heritage Statement setting out the significance of the existing windows and the justification for the proposal is expected with any application, and permitted development rights in a listed building curtilage covers what else is restricted.

Building regulations and thermal requirements

Planning and Building Regulations are separate, and window replacement engages the second one even where the first is silent.

Replacement windows and doors are controlled fittings, so thermal performance requirements apply. Approved Document L sets the standard for replacements in existing dwellings, and it has been tightened more than once, so check the current edition rather than a builder's recollection. Background ventilation has to be maintained or improved when windows are replaced, which is why trickle ventilators appear in specifications that did not previously have them.

Two further requirements catch people. Where a window serves a room used for sleeping, escape window sizing under Approved Document B applies and a replacement must not reduce the escape provision. And where the sill is low, guarding requirements apply.

Most replacement work is self-certified by an installer registered under a competent person scheme, which produces a certificate the conveyancer will ask for. Work done by an unregistered installer needs a building control application, and a retrospective regularisation is slower and more expensive than doing it properly.

Keep the certificates with the deeds along with any planning decision. The most common problem at sale is not an unlawful window. It is a lawful window with no paperwork.

You can draft the supporting statement for a window or door application in PlanScope in minutes, including the Planning Statement and any heritage narrative, then send it for professional review before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that works are lawful.