Replacing a double-glazed window typically takes between 30 and 90 minutes per window for a standard-sized unit, according to the Fenestration Self-Assessment Scheme (FENSA, 2026). This timeframe covers removal of the old sealed unit and installation of the new one by a certified installer.
The actual time depends on whether you are replacing just the glass sealed unit (a “glass-only” replacement) or the entire window frame and sash. A glass-only swap is faster, often 20–40 minutes, while a full frame replacement can take 1–3 hours per window. Access, weather, and the condition of the existing frame also affect the job duration.
Glass-only replacement is the quickest option
If the window frame is sound and only the sealed glass unit has failed (e.g., misting or condensation between panes), a glass-only replacement takes around 30 to 60 minutes per window. The installer removes the old unit, cleans the frame, and fits the new sealed unit with fresh gaskets. This avoids the need to remove or replace the outer frame, which reduces labour time and cost. Energy Saving Trust notes that a failed sealed unit significantly reduces thermal performance, so prompt replacement is advisable (Energy Saving Trust, 2026).
Full frame replacement takes longer
Replacing the entire window, including frame and sash, typically takes 1 to 3 hours per window. This involves removing the old frame, checking the structural opening, fitting the new frame, sealing it, and reinstalling the sash. For a typical three-bedroom house with 10 windows, a full replacement job may take 1–2 days. The work must comply with Building Regulations, and installers registered with FENSA or CERTASS can self-certify the work (GOV.UK, Approved Document L, 2026).
Factors that can extend the timeline
Access difficulties, such as windows on upper floors requiring scaffolding, can add 1–2 hours per window. If the existing frame is rotten or the brickwork is damaged, additional repairs may be needed, extending the job by 30 minutes to an hour per window. Weather also plays a role, rain or high winds can slow sealant curing and safe installation. The Microgeneration Certification Scheme recommends checking installer credentials to ensure efficient, compliant work (MCS, 2026).
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The existing aluminium frame is removed, the brickwork opening is checked for damp, and a new uPVC frame is fitted with expanding foam insulation and new silicone seals. According to the Energy Saving Trust, replacing a single failed unit in a 3-bedroom semi can save roughly £110 per year on heating bills, as the new window eliminates draughts and improves the U-value from around 3.0 W/m²K to 1.2 W/m²K. The payback period for this single window is therefore under 8 years, and over a 25-year lifetime the savings total approximately £2,750. If the whole house had 10 similar windows replaced, the combined job would take around 20 hours across two days, with a total cost near £8,500 and annual savings of £1,100.
| Item | Figure |
|---|---|
| Upfront cost after grants | £850 |
| Yearly savings | £110 |
| Payback period | 8 years |
| 25-year lifetime savings | £2,750 |
What homeowners often get wrong
The most common mistake is assuming all double-glazed replacements take the same time, when the difference between a glass-only swap and a full frame job can be over an hour per window. Here are three frequent errors that cost time and money
- Choosing glass-only when the frame is failing Homeowners often request the quicker glass-only replacement to save time, but if the existing frame is rotten, warped, or over 20 years old, the new sealed unit will fail prematurely. The right answer is a full frame replacement, which takes 1–3 hours but prevents a repeat job within 5 years, potentially wasting £400 on a second call-out.
- Ignoring access issues until the installer arrives Many people assume the job will take the standard 30–90 minutes, but they forget to clear furniture, remove curtains, or ensure the installer can reach the window from outside. The right answer is to prepare the area beforehand, as a blocked pathway or locked gate can add 30 minutes of setup time and an extra £50 in labour charges.
- Skipping the trickle vent check Homeowners often replace the window without checking if building regulations require a trickle vent, especially in bedrooms or rooms with gas appliances. The right answer is to confirm with the installer that the new unit includes adequate ventilation, because a missed vent can void the warranty and lead to condensation issues that cost £200 to rectify later.
Quick reference
- A standard glass-only replacement for a 1.0m x 0.8m window takes 20–40 minutes, while a full frame replacement for the same size takes 1–3 hours.
- Replacing all windows in a typical 3-bedroom semi-detached house takes 2–3 days for a full frame job, or 1 day for glass-only swaps.
- You must use a FENSA-registered or CERTASS-registered installer to comply with UK building regulations and avoid a £400 enforcement fee from the local authority.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- If the existing frame has any rot, rust, or damage beyond surface wear, a full frame replacement is mandatory to maintain the 10-year insurance-backed guarantee on the sealed unit.
Frequently Asked Questions
A glass-only replacement takes around 20 to 40 minutes per window. FENSA states this is the fastest option when the frame remains sound.
A full frame replacement typically takes 1 to 3 hours per window. The Energy Saving Trust advises factoring in time for structural checks and sealing.
Yes, adverse weather can add 30–60 minutes per window. Installers may need to protect the opening and use additional sealants, according to FENSA guidelines.
