Under the Building Act, replacing windows in an existing house is maintenance that needs no decision on the building intent and no notification. Two exceptions apply (§ 18 para. 4 points c and d). One is maintenance of a monument. The other is maintenance that could affect stability, fire safety, the look from public space or the environment. In a heritage area the Monuments Act also applies (49/2002 § 32). Below we show how to compare quotes. Installation decides much of the result.
Do I need a notification or a permit to replace windows?
Act 25/2025 Coll. no longer has a building permit. Replacing the infill of openings, meaning windows and doors, is listed in § 18 para. 5 point a) item 1 as maintenance. Listed maintenance needs neither a decision on the building intent nor a notification. Replacing blinds, roller shutters and shading screens is on the same list.
A notification is required, however, for maintenance that could affect stability, fire safety, the look in public space or the environment (§ 18 para. 4 point c). If the replacement changes the openings or the street facade, ask the building office which procedure applies before you order.
Where a notification is needed, the office handles it within 30 days by verifying the project for notification and issuing a verification clause. The deadline is in § 63 para. 6. For an office that stays silent, § 63 para. 7 words the presumed issue of the clause for a notified minor structure or building alteration, not for maintenance. Notified works may start within three years of verification (§ 63 para. 10). The glossary entry building alteration notification describes the procedure.
What changes for window replacement from 1 January 2027?
Amendment 243/2026 Coll. changes § 18 from 1 January 2027. A notification will be needed for maintenance that may affect stability, fire safety or the environment. The look from public space drops out of the text. New is a notification for maintenance of a monument. A building in a heritage area needs one if the maintenance changes its outer appearance (§ 18 para. 4 points f and g in the new text).
From 2027, replacing windows in a heritage area in a way that changes the facade will therefore also need a notification under the Building Act. The text above applies to the end of 2026. This article reflects the law on 11 November 2026.
What applies to a monument or a heritage area?
The owner of a cultural monument must ask the regional heritage office for a decision on the restoration intent before work starts (Act 49/2002 Coll. § 32 para. 2). Under § 32 para. 1, restoration also covers activities that need no notification or permit under the building rules. Replacing windows on a monument is therefore a matter for the heritage office before the building office.
A non-monument in a heritage area needs a decision first. The owner asks the regional heritage office for a decision on the intent of the alteration (§ 32 para. 5). In a heritage area, the owner submits documentation to the regional heritage office before any restoration or alteration. The office issues a separate binding opinion on it (§ 32 para. 10). A property in a protection zone that is no monument needs a binding opinion from the heritage office before the alteration (§ 32 para. 11). See the entries heritage protection zone and heritage authority binding opinion.
When is replacing windows a major renovation of a building?
Act 555/2005 Coll. on the energy performance of buildings defines a major renovation. It is building work on an existing building that touches more than 25 % of the envelope area. The act names insulation of the outer and roof envelope and replacement of the original window and door infill as examples (§ 2 para. 7). It may be done at once or in partial steps.
After a major renovation, the building should meet the minimum energy performance requirements for a new building. This applies where it is technically, functionally and economically feasible (§ 4 para. 1). The designer includes compliance in the project documentation and states the result of the energy assessment in the technical report (§ 4 para. 4). The technical parameters come from technical regulations and standards, not from a supplier's quote. From 1 January 2027, Amendment 246/2026 Coll. changes the text of these provisions. If you plan several steps, see deep renovation.
Which window parameters should you compare in quotes?
Compare the parameters of the whole window as set in the project, not only the advertised value of the glass. A lower U-value means less heat flow. The US Department of Energy (DOE) lists the features that lower it. This is the physics of glass and frame, not a Slovak rule.
- Glazing. More panes, a low-emissivity (low-e) coating and a gas fill between the panes, argon or krypton, lower the U-value. See double glazing, triple glazing and glazing U-value.
- Frame. Frame and sash should be made of a material that conducts heat poorly, such as wood, vinyl or fiberglass. A mostly metal frame needs a thermal break. See frame U-value.
- Spacer. It holds the panes at the right distance, reduces heat flow and helps prevent condensation.
- Number of panes. The designer chooses double or triple glazing against the target from the energy assessment. We give no numeric values for them here because we have no dated source.
Why does the installation of the window matter?
Even very efficient windows can cause draughts and moisture condensation if they are installed badly. DOE advises following the manufacturer's instructions and hiring trained installers. According to DOE, gaps around windows and doors are among the most obvious air leaks. Air leaks also let moisture into the structure, which can damage wood and lead to mould.
After window replacement the house is more airtight. According to DOE, without proper ventilation excessive water vapour can build up in a home. Plan ventilation together with the windows. The position of the window in the wall and its connection to the insulation are settled in the design, because a thermal bridge forms there. If the facade is also being insulated, ask the designer for the connection detail before you order the windows. The related procedure is in the article on facade insulation.
What rules apply to installing a window?
The Slovak Innovation and Energy Agency (SIEA) lists rules for installing windows well in its 2018 brochure on insulation and window replacement. They are recommendations of the agency, not a regulation.
- Anchor the window mechanically, with screws or anchoring devices. Foams and adhesives are not enough.
- Fill the whole gap between the window and the structure with insulating foam.
- The layers in the gap must let water vapour escape outward. Use a vapour-tight tape on the inside and a waterproof but vapour-open tape on the outside.
- Insulate the reveals and the lintel up to the window frame to limit thermal bridges. Also insulate the part of the sill under the outer sill board.
- Do not apply plaster directly to the sealing foam. It does not protect the foam enough against moisture from the room.
In what order do you replace windows and insulate the facade?
SIEA advises replacing the windows just before insulating the building envelope. The replacement works on the external walls. If the windows change only after insulation, the insulation of the reveal and the lintel would be damaged.
The agency calls replacement after insulation a relatively high risk. The insulation system may be damaged, and the joint between the window, the insulation and the plaster will never be ideal. If a fault shows, for example as a leak, SIEA says problems with the warranty on the building work usually follow. A detailed project with drawn details and an order of steps should lead the renovation: windows first, then insulation of the walls and reveals.
Why can mould appear after a window replacement?
New windows are tighter, so air exchange falls and humidity in the house can rise. Mould then forms at thermal bridges. SIEA states this for older houses where insulation did not follow the new windows. Mould can appear there after one or more winters, even where there was none before.
Insulation of the external walls from outside, including the reveals, lintels and sills, raises the temperature on the inner surface of the walls. This prevents condensation, provided the house is ventilated enough and regularly. If the masonry holds moisture from rain or the ground, remove its source first. Repair the roof and gutters, lead rainwater away from the foundations and give the ground-bearing floor a new waterproofing layer.
How do you ventilate after a window replacement?
After a window replacement, keep the required ventilation rate. SIEA states that as buildings get tighter, the need for controlled ventilation with small air-handling units grows, and that heat recovery is worth considering. According to the agency, some manufacturers offer windows with integrated ventilation and heat recovery in the frame.
Ask the designer to assess controlled ventilation for the renovation.
Will the Obnov dom programme support window replacement?
The Obnov dom mini+ call supports new windows and doors. It also supports insulation of the external walls, the floor of an unheated attic and the ceiling of an unheated cellar. On 11 November 2026 the programme page states a grant of up to 11,000 euro for owners of family houses. Household income per member for 2024 must not exceed 12,990 euro, with exceptions for some groups. The page expects a second round in January 2027, as an estimate.
The measures page gives no technical parameters for windows and does not say whether costs incurred before the contract count. Read the conditions of the current call before you order. The programme is described in the Obnov dom subsidy guide.
What should a window quote contain?
- The window parameters match the project and the energy assessment.
- The quote describes the installation and refers to the manufacturer's instructions.
- It is clear who handles reveals, sills and the connection to the insulation.
- The price includes removal of the old windows and repair of the inner reveals.
- A trained installer does the work.
We give no window prices in euro. They depend on manufacturer, size and hardware, so ask for several concrete quotes for the same parameters. Planning the whole sequence of measures is covered in the article on deep renovation.