Bay windows: cost, planning and building regulations
Replacing a standard uPVC bay window typically costs about £1,200 to £3,000 in 2026, and many homeowners pay around £2,000. Building a new bay where there wasn’t one is a bigger structural job, often £3,500 to £8,000, and in England it usually needs planning permission.
Typical bay window prices
| Job | Typical price |
|---|---|
| Replace a three-panel uPVC bay | £1,200 – £2,000 |
| Replace a uPVC bay (wider range, larger bays) | £1,220 – £2,950 |
| New structural bay, including lintel | £3,500 – £8,000 |
Prices rise with more panels, curved (bow) designs, timber or aluminium frames, and any structural work such as a new lintel or roof over the bay. Quotes don’t always include plastering, brickwork repairs or redecorating.
Do you need planning permission? (England)
- Replacing existing windows, including a bay, with similar-looking ones is normally permitted development for a house.
- A new bay that sticks out from the front of the house is an extension forward of the main elevation facing the road, which permitted development doesn’t allow, so it usually needs planning permission.
- Listed buildings need listed building consent, and conservation areas or Article 4 directions can restrict even replacement windows.
Building regulations
- Replacement windows in England and Wales must meet building regulations, including energy efficiency. In England, a replacement window in an existing home should achieve a whole-window U-value of 1.4 W/m²K or better (or an equivalent energy rating).
- Use an installer registered with a competent person scheme such as FENSA or Certass, who can self-certify the work. Otherwise, you need to apply to building control yourself.
- Other rules can apply too: escape windows on upper floors, safety glazing low down, ventilation, and security standards for new homes.
- A new structural bay also needs building regulations approval for the lintel, foundations and any roof.
Wales, Scotland and Northern Ireland
The planning position and the 1.4 W/m²K figure above are England’s. FENSA and Certass certification is for England and Wales.
- Wales has its own permitted development rules. They are similar to England’s in many places but not identical, so check with your council before relying on English limits.
- Scotland has separate planning rules, and most building work needs a building warrant from the council’s building standards team before it starts.
- Northern Ireland has its own permitted development order and building regulations, run by local councils.
Before you hire someone
- Decide whether it is a replacement (similar look) or a new bay, because the rules differ.
- Use a FENSA or Certass registered installer and get the certificate afterwards.
- Check the quoted U-value or energy rating, and the glass and frame specification.
- Ask whether the bay roof, lintel or brickwork need work, and whether that is included.
- Ask what making good inside (plaster, sills, decorating) is included.
- Check guarantees, and whether they are insurance-backed.
Common questions
How much does a new uPVC bay window cost?
Replacing a standard three-panel uPVC bay typically costs about £1,200 to £2,000, with larger bays up to about £3,000. A new structural bay where there wasn’t one usually costs £3,500 to £8,000.
Do I need planning permission for a bay window?
Replacing an existing bay with a similar one usually doesn’t need permission for a house in England. A new bay projecting from the front of the house usually does.
What U-value do replacement windows need?
In England, replacement windows in existing homes should achieve a whole-window U-value of 1.4 W/m²K or better, or an equivalent energy rating. A FENSA or Certass installer can certify this.
Who certifies replacement windows?
In England and Wales, installers registered with a competent person scheme such as FENSA or Certass can self-certify that the windows meet building regulations. Otherwise you apply to building control.
Sources
Figures and rules were checked on 14 October 2026. Prices are published UK ranges, not quotes: get written quotes for your own job.