Replacing windows in an old Brussels house: permits, EPB, ventilation and costs

Your windows are draughty, the single glazing streams with condensation in winter, or you have a quote to replace the lot in PVC. Before you sign, three questions settle the matter. Do the windows need replacing, or can they be renovated? Does the facade in question need a planning permit? And how will you ventilate a house that is about to become more airtight? This guide sets out what Homegrade (the free information centre for private households), urban.brussels, the municipality of Uccle (Ukkel in Dutch) and Buildwise say on each point, with the date of every source. It gives no prices in euros, because we found no dated official source for them; the reason is explained further down.

Key points

• According to Homegrade, replacing windows that are still in good condition generally makes little sense, either economically or environmentally; renovating the old ones should be considered first, especially if they have heritage value.
• Replacing windows "like for like" (original shapes and architectural appearance kept, which Homegrade interprets as the same material, the same colour and the same profiles) is exempt from a planning permit for a building that is not protected. According to Homegrade, going from white timber to white PVC needs a permit, even if the result looks identical.
• Listed building ("classé") or one on the safeguarding list (liste de sauvegarde): according to Homegrade, a permit is always required. The 2008 order does, however, exempt like-for-like window replacement on the unprotected parts of a protected building (art. 35/22, 5°). In the protection zone of a listed building, or less than 20 metres from a protected building, some exemptions do not apply.
• Insulation, airtightness and controlled ventilation go together, Homegrade points out: with more airtight new windows, plan the ventilation before they go in, not after mould appears.

Keep, renovate or replace: the first question

The Homegrade brochure of December 2022 takes the questions in this order. People mostly replace windows for thermal and acoustic comfort, but before choosing that route you should check whether keeping the existing windows would be better, for example to preserve the look of an old facade. The brochure adds that old windows that have been maintained regularly are often still in good condition after more than a hundred years, and that several techniques can improve their thermal and acoustic performance.

The Guichet Energie (energy desk) of the municipality of Uccle, in an undated information sheet, puts forward two options. One is to replace single glazing with double glazing while keeping the original look, which it describes as expensive but effective. The other is to renovate the old windows with new EPDM seals, provided the frames are of good quality (the sheet cites old oak windows as an example), which it describes as cheaper and less demanding on resources.

On payback, Homegrade is cautious. Payback periods vary widely with energy prices and the cost of the works, and the figures given are only indicative. For simply shaped single-glazed windows replaced with insulating glazing, the brochure gives a payback of 20 to 30 years, based on the expected energy savings. For windows with complex shapes (arched heads, glazing bars) or windows that already have double glazing, the payback period often exceeds the life of the window.

So before asking for a replacement quote, have the actual condition of the windows assessed. Homegrade offers brochures on old timber and steel windows for this, as well as free advice.

When do you need a planning permit?

As a general rule, renovation works need a permit and the exemptions in the order of 13 November 2008 are the exception; they do not apply if the work departs from a land-use plan, a planning regulation or a subdivision permit (permis de lotir) (the principle, the procedure and the time limits are explained in our guide to the planning permit in Brussels).

For windows, the most common situations can be summarised as follows, based on the Homegrade page updated on 10-08-2026 and the information sheet of January 2024:

Two of Homegrade's points carry a lot of weight in an old house. First, "like for like" refers to the legal situation (situation de droit), meaning the legal situation officially recognised by the municipality, whose plans are normally held in the planning archives, and not necessarily the situation on the ground. The brochure advises checking with the municipality's planning department whether the existing windows can serve as the model, or whether they have already been replaced in breach of the rules. Second, the information sheet points out that the regulations are often hard to interpret: always check with the municipality.

The municipality of Uccle, on its "Introduire une demande" page, lists window replacement among the works that need a permit if the appearance changes (materials, division, shape, colour), and adds that the municipal board (Collège échevinal) encourages the installation of timber windows. For Forest, Ixelles and Watermael-Boitsfort, ask the municipality's planning department: a municipal regulation or a PPAS (special land-use plan) may add conditions.

One more point from the Homegrade brochure: in a block of flats, the windows are usually listed in the basic deed (acte de base) as private parts, but the building's appearance is common property. The basic deed therefore generally requires their replacement to be approved by the co-owners, and it may set rules on materials, colours and shapes.

Protected buildings and the surroundings of listed buildings: separate rules

For a building that is listed or on the safeguarding list (liste de sauvegarde), the Homegrade brochure says that a planning permit is always required and that your point of contact is the Region's Cultural Heritage department (Direction du Patrimoine culturel). The text of the 2008 order is more nuanced. It exempts like-for-like window replacement (original shapes, arched heads, visible divisions, opening and fixed parts kept, architectural appearance unchanged) on the unprotected parts of a protected building (art. 35/22, 5°), but not on the protected parts. Ask the Direction du Patrimoine culturel to confirm which parts are protected.

The procedure specific to protected buildings (a "permis unique", or single permit, issued by the "fonctionnaire délégué", the regional planning official who decides in certain cases; time limits; the opinion of the Royal Commission for Monuments and Sites, CRMS) is described by patrimoine.brussels and summarised in our guide to Brussels character houses.

The surroundings of a listed building are less well known. According to the Homegrade information sheet, the exemption for altering openings does not apply if the facade lies within the protection zone of a listed building or one in the listing procedure. According to Homegrade, other exemptions (the colour of a gable wall, or insulating a wall or a pitched roof, for example) no longer apply to a building less than 20 metres from a protected building. Patrimoine.brussels asks you to contact its services before starting works on a protected building or on a building within the protection zone of a listed building.

The municipality of Uccle notes that there are many buildings and sites on its territory protected by listing or by the safeguarding list: before you order windows, check the status of your house (see the question "Is my house protected?" below).

Putting together an application without an architect

When a permit is needed for windows, it can often be applied for without an architect. The Homegrade brochure states that this is possible as long as the changes involve no structural work (such as altering a lintel or widening an opening). Homegrade's tutorial on replacing windows and doors lists the documents for this kind of application:

• a location plan and a site plan, both available from gis.urban.brussels;
• a photo report: four colour photos of the facade, one taken straight on and others that include the neighbouring buildings, numbered and marked on the site plan;
• the plan of the street facade in its legal situation (scale 1:50), to be requested from the municipality, or, if none exists, a plan of the existing situation that you have drawn up;
• the plan of the proposed situation (scale 1:50), with a key giving the materials and colours of the existing and the altered windows;
• the planning permit application form.

If the plan of the legal situation differs from the existing situation, a plan of the actual situation may also be required. A draughtsperson or an architect can help with the plans, but in this case you do not have to use one.

Why all this care? Because the municipality of Uccle puts it bluntly: once an offence has been recorded in an official report, you have to apply for a regularisation permit whose outcome is uncertain, and a fine is payable in any case, even if the offence has been regularised. It notes that some people believe they are within the rules because a contractor told them no permit was needed, and recommends checking with the planning department.

Comparing windows: glazing, frames, installation

Once the permit question is settled, Homegrade suggests choosing according to the performance you expect, which depends on the quality of the frame profiles, the glazing, the seals and the hardware, and also on how carefully the windows are fitted. A few reference figures from the brochure:

The brochure also gives the range for so-called "high-performance" glazing: from 1.1 W/m².K (double glazing) to 0.6 W/m².K (triple glazing). It warns against chasing the lowest figure: going from a Ug of 1.1 to 1.0 loses 12% of solar gain and 10% of light transmission for a tiny gain in insulation, which can matter in homes built before the 1960s, where glazed areas are limited.

Other points from the brochure that help when reading a quote:

• Light. Glass makes up about 70% of a typical timber window; Homegrade advises against profiles wider than the original ones.
• Airtightness. The classes run from 1 to 4; class 3 is in principle enough for urban renovation, but most new windows are class 4, which the brochure recommends choosing.
• Acoustics. Homegrade mentions high-density timber windows (600 kg/m³ minimum) and asks for three rebates and two seals; PVC and aluminium can also be suitable, preferably with acoustic test results.

EPB and ventilation: what a more airtight window changes

Two things are at stake here, and both need preparing before installation.

EPB rules for works. According to the Homegrade brochure, when window replacement requires a planning permit, the EPB (energy performance of buildings; PEB in French) regulations apply: the Ug value of the glazing must be no higher than 1.1 W/m².K and the Uw value of the windows (weighted average) no higher than 1.5 W/m².K. In the same case, the 2022 brochure states that the EPB regulations require air inlets in the so-called dry rooms (living room, bedrooms and so on) where windows are replaced. These inlets can be natural (a vent in the window frame or in the facade) or mechanical (balanced, double-flow ventilation). According to the Bruxelles Environnement vade mecum on EPB requirements for works (page reviewed by the administration on 17-09-2026), for permit applications from January 2023 onwards the requirement covers every existing room where windows are replaced, added or removed (air supply or extraction), and a minimum flow rate of 45 m³/h per linear metre of replaced or added window is accepted as an alternative to the flow rates for new rooms. This guide does not deal with the EPB certificate; for the exact requirements of a given project, the Homegrade information sheet refers you to Bruxelles Environnement.

Ventilation. Homegrade's brochure on ventilation in renovation explains that indoor air picks up water vapour (breathing, cooking, showers, laundry) and that this vapour can condense on cold surfaces such as single glazing or a poorly insulated external wall; over time, damp and condensation damage materials and encourage mould. It adds that insulation, airtightness and controlled ventilation go hand in hand. Buildwise points out that ventilation through air leaks is uncontrolled, so it is sometimes too little and sometimes too much.

For windows, the brochure on replacement gives some useful details:

• if continuous ventilation is not provided by a system independent of the windows (vents in the external walls, double-flow ventilation), air inlets must be built into the windows;
• discreet devices fitted above the top rail of the frame are preferred; those placed within the frame reduce the glazed area, are very visible and weaken thermal and acoustic performance further;
• hardware that lets the window open very slightly to admit air does not comply with the EPB regulations;
• the brochure on ventilation states that the tilt position of a tilt-and-turn window is not regarded as an adjustable air supply opening.

A safety point from the ventilation brochure: open-flued combustion appliances (stoves, open fires, atmospheric water heaters and boilers) draw their combustion air from the room, and an insufficient air supply increases the risk of carbon monoxide (CO) being produced. If you have such an appliance, follow the specific standards and have this checked before making the room more airtight.

If you are also insulating the walls from the inside, Buildwise construction detail no. 1427 (13-10-2022) stresses continuity between the wall insulation and the window insulation (insulation returned under the sill board, along the rebate and under the lintel) to avoid condensation at the junction. To work out where damp that appears after the works is coming from, see our guide to damp in a Brussels house.

What does it cost, and what support is there?

We do not publish prices in euros for windows. The official sources we consulted (Homegrade, the municipality of Uccle) do not give any, and we found no dated price range in any readable institutional source; a price per square metre also depends on the material, the shapes (arched heads, glazing bars), the installation and the condition of the openings. What the official sources do say:

• For comparable technical quality, Homegrade ranks the materials as follows: PVC is generally the cheapest option, then timber (with variations depending on the species), aluminium and finally steel.
• It recommends being wary of windows sold at "rock-bottom prices", which often mean lower quality across all the components.
• It points out that the contractor who fits the windows is subject to ten-year liability (responsabilité décennale).

To compare quotes, ask for each one to state the material, the Ug and Uw values, the airtightness class, the air inlets, the installation and who handles the permit if one is needed.

Rénolution grants. As of 19 October 2026, be.brussels states that there is no government decision on any RENOLUTION grants for 2025 and/or 2026, and that no application can be submitted for final invoices dated 2025 or 2026. Do not build a budget on a grant that is not open: the guide Renovation grants in Brussels: what is left of Rénolution? gives the full picture and lists the support that is still available.

The other support listed by the renolution.brussels portal (6% VAT, the ECORENO loan, free advice from Homegrade) has its own conditions, presented in our guide to Rénolution grants: check them with each organisation before you sign a quote.

Frequently asked questions

Can I replace my timber windows with PVC without a permit?

No, according to Homegrade: replacing white timber windows with white PVC requires a permit, even if the shape and colour are identical, because the material changes. According to Homegrade, a replacement is exempt only if the material, shapes, colour and moulding detail of the profiles are kept; the text of the order refers to original shapes and an unchanged architectural appearance.

What about a window at the back, not visible from the street?

Creating, closing up or altering openings that are not visible from the public space is exempt from a permit if the works involve no structural work and the facade is not in the protection zone of a listed building or one in the listing procedure (Homegrade, art. 21, 8°). If in doubt, ask the planning department to confirm.

Are trickle vents in the windows compulsory?

According to Homegrade, when the replacement requires a permit, the EPB rules require air inlets in the dry rooms where windows are replaced (2022 brochure). According to the Bruxelles Environnement vade mecum, for permit applications submitted since January 2023 the requirement covers every existing room concerned, with air supply or extraction depending on the room; these devices go either in the windows or the facade, or are mechanical. In every case, the brochure reminds you that continuous ventilation must be provided.

Is my house protected?

Check the list on patrimoine.brussels or the map on gis.urban.brussels ("patrimoine" layer), as Homegrade advises, then confirm with the municipality. The statuses (listing, the safeguarding list, the inventory) and what they change are explained in our guide to Brussels character houses.

What is the risk if I fit windows without a permit?

According to the municipality of Uccle, once an offence has been recorded in an official report you have to apply for a regularisation permit with an uncertain outcome, and a fine is payable even if the offence is regularised. The criminal and administrative consequences of an offence are explained in our guide to the planning permit in Brussels.

On 03-10-2026 the editorial team reread the Homegrade brochure on replacing windows (December 2022), the brochure on ventilation in renovation (February 2020), the Homegrade page on exempt works (updated on 10-08-2026) and its information sheet of January 2024, the tutorial on permits without an architect, the municipality of Uccle's pages (planning, offences, Guichet Energie information sheet), the patrimoine.brussels page on works to a protected building, two Buildwise pages, and the be.brussels and renolution.brussels pages on grants. Several of these documents are old or undated, and we cross-checked the articles cited against the consolidated text of the 2008 order (Justel, text incorporating the order published on 10-08-2026): statuses, thresholds and requirements change, so check with your municipality's planning department and with Homegrade before deciding on any works.