How much is a bay window for a semi?

Roughly £1,500 to £3,000 fitted, because the frame is larger, there are more corners, and it frequently carries structural load that has to be supported during the work.

Can I keep my bay and only replace the flat windows?

Yes, and it is common where budget is tight. The bay is usually the most expensive single opening, so leaving it until later spreads the cost. The appearance will be mixed until it is done.

Do I need scaffolding for a semi?

Often yes for the upper floor, though some installers work from a tower where access allows. Ask whether scaffolding is included or extra, because it is a common add-on.

Costs by property

Cost of new windows for a 3-bed semi

A typical three-bedroom semi-detached house has around nine windows, which in uPVC comes to roughly £3,600 to £7,650 supplied and fitted. Most have a bay to the front, which is priced separately and can add £1,500 to £3,000 because it is usually structural.

The three-bed semi is the default British house and the one most window quotes are written for. It also has a specific feature that catches people out.

Worked example

9 windows

uPVC: £3,600 – £7,650
aluminium: £5,400 – £11,250
timber: £7,200 – £14,400

£3,600 – £7,650

supplied and fitted, uPVC

These figures come from published industry price guides, not official statistics, and several of those sources are installers or comparison sites with a commercial interest. They are a starting point for a conversation, not a quote. Your installer sets the actual price after measuring.

Costwise UK new windows cost guide · checked 2026-10-09
Checkatrade double glazing cost guide · checked 2026-10-09

The bay is the variable

Most inter-war and post-war semis have a bay to the front, and in a lot of them that bay carries the wall above it. Removing the old frame removes part of that support, so the replacement needs either a structural frame or separate jacking posts built into the corners.

This is why two quotes for the same house can differ by thousands. One has priced the bay as a window and the other has priced it as a structural job. Ask directly how the load above the bay will be carried.

A typical schedule

A nine-window semi usually breaks down as a bay to the front, one or two further ground-floor windows, three bedroom windows, a bathroom window and a landing window. Half of those are small, which is why an average price per window is a poor guide.

Ask for the quote itemised opening by opening. It is the only way to see whether the cheap quote has used fewer openers or thinner glass.

Access and the side passage

A semi has one side elevation and usually a narrow passage down it. If that passage is too tight for scaffolding, the rear elevation may have to be worked from the garden with a tower, which adds time.

It is worth mentioning the access at the point of enquiry rather than on the day, because it changes the plan rather than the price list.

Questions

How much is a bay window for a semi?: Roughly £1,500 to £3,000 fitted, because the frame is larger, there are more corners, and it frequently carries structural load that has to be supported during the work.
Can I keep my bay and only replace the flat windows?: Yes, and it is common where budget is tight. The bay is usually the most expensive single opening, so leaving it until later spreads the cost. The appearance will be mixed until it is done.
Do I need scaffolding for a semi?: Often yes for the upper floor, though some installers work from a tower where access allows. Ask whether scaffolding is included or extra, because it is a common add-on.