How much does it cost to replace two rotting windows in Britain in 2025?

More than you might think

I recently met Chris Howell, an accountant who owns a small flat in London. He told me an absolutely infuriating story.

His windows are in a state. The frames are rotting and it’s only a matter of time before they give out. They’ve had a good innings – nearly 100 years of service – but it’s time to swap them out for new double-glazed windows.

Swapping single for double-glazed windows is one of the easiest ways to cut down your heating bills. Sky-high energy bills mean the investment usually pays for itself in a few years.

Not for Chris however. Chris lives in a block of flats in Westminster and that’s a big problem.

Planning Permission

Westminster Council require a planning application for non-like-for-like replacements. Applying will cost him £528. Chris isn’t a planning lawyer and having seen attempts by neighbours to apply for permission reckons he’ll need to hire a planning consultant (+£1,000).

Now, about the windows themselves. Chris got a quote for uPVC replacements (£2,500), but having looked at similar applications he’s convinced the council won’t allow it. Instead, he will need to pay extra (£5,000) for aluminium-frame windows for a closer aesthetic match.

Chris, it should be said, wasn’t too fussed about having to get the more expensive ones. He reckoned the freeholder might insist on them anyway. The thing that bothered him was the extra costs associated with obtaining planning permission.

Building Safety Regulator

Yet, the story doesn’t end there. Chris lives in what is known as a ‘Higher Risk Building’. This was a category created by the Building Safety Act and includes all buildings 18m or higher. The block of flats Chris lives in is slightly taller than 18m so any major works in his building need special approval.

In theory, this should be easy. The system was meant to outsource approvals for things like window replacements to a ‘Competent Persons Schemes’. In other words, have your window installed by a regulated professional and they can sign-off on building control. Easy peasy.

The problem is Chris asked every single scheme qualified to sign-off the works (there are three) and they all told him the same thing: they won’t sign off any work on a higher risk building. It is not clear why or if the Government engaged with the schemes.

In fact, it is not entirely clear that the Building Safety Regulator even knew about this issue. They were quoted in The Standard saying: “However, most window replacements in higher-risk buildings (HRBs) do not need building control approval because they can be signed off through Government-recognised competent person schemes.” It isn’t clear why they said they couldn’t sign-off on work in HRBs.

The lack of competent person schemes willing to sign-off on his window replacement (or indeed any window replacement) means that Chris had no choice but to go to the Building Safety Regulator directly.

In theory, it should be straightforward. Send off an application then in 8 weeks get a response given the go ahead. However, Chris was wary about applying without help. First, the BSR has rejected 70% of applications submitted to it. Second, even extremely straightforward cases, like a flatowner renovating their kitchen (and in the process moving a fire door) have dragged on.

The BSR charges a flat £288 application fee and then additional fees of £144 per hour of time spent on the application. While there is an 8 week deadline, the BSR often asks for extensions. One flatowner has been waiting for almost a year for a decision on minor flat renovation. She recently told The Telegraph it cost her £4,000 in fees. Given the complexity, Chris is looking into hiring a consultant to write the application for him. One quoted for £12,000, another for a more reasonable £5,000 (+5,000). In other words, the same cost as the windows (plus installation).

Chris told me that he estimates there are around 700,000 flats in higher-risk buildings. Yet, only 100 applications relating to windows have been submitted to the Building Safety Regulator. Either people are put off by the cost, like Chris, or are just ignoring the rules. If it’s the latter, then they could face up to two years in prison (and a big fine).

So far, I have neglected to mention a key fact about Chris’s flat: it is on the ground floor.

The BSR has a massive backlog – it is one of the big reasons why housebuilding in London has collapsed – do we really want minor works to ground floor flats clogging it up.

The cost of replacing two-windows in Westminster in 2025

Add it all together and the cost of replacing two rotten windows with new double-glazed windows is £15,816.

That’s £5,000 for the windows themselves and just over £10,000 spent on persuading the state to let him put them in.

Chris, for obvious reasons, has no plans to pay £15,000 to replace his windows. He’s holding tight for now in the hope the law changes.

What should be done?

It shouldn’t cost £15,000 to replace two rotting single-glazed windows with modern double-glazing.

When people are struggling to heat their homes, we shouldn’t throw extra barriers in the way of basic energy efficiency measures.

Likewise, it is a bit hypocritical of Westminster Council to declare a ‘Climate Emergency’ and set a target of Net Zero by 2040, and then make the greener option more expensive than necessary.

I sympathise with Westminster Council’s desire to stop ugly plastic windows from spoiling well-kept areas, but there’s a better option.

The neighbouring borough of Kensington and Chelsea have put in place a Local Development Order for new ‘like-for-like’ double-glazing. It effectively grants automatic planning permission to any resident wanting to swap their single-glazed windows out for double-glazed ones so long as the new windows match the appearance of old. The more expensive windows with metal frames that Chris was resigned to purchasing would qualify, while the cheaper plastic ones wouldn’t.

That’s planning sorted, so what about the building safety regulator?

The quickest fix would be to get the three competent persons schemes to agree to approve work on higher-risk buildings. It is unclear why they don’t, but the job of MHCLG should be to persuade them to get onboard. And if necessary, legislate to remove any legal barriers.

The Building Safety Regulator is an extremely flawed organisation with an extremely wide mandate. The Centre for Cities notes the model of a national building safety regulator is unique (at least outside city states). There is a debate to be had over whether the job of building control should be handed back to local authorities and simply resourcing them properly. The Building Safety Regulator could still exist in more of an advisory form. This would be controversial and the Government has shown no appetite for a radical move despite the collapse in London housebuilding.

Much less controversial is the idea that the Building Safety Regulator is forced to do too much and should focus its attention on the most complex and highest risk cases.

In fact, in a recent interview, Andy Roe, the fire commissioner turned Building Safety Regulator Chair, voiced his support for lightening the BSR’s load and returning the job of approving/blocking minor works to councils.

“You’ve got to put your resources where the risk is the greatest. The Building Safety Regulator is no different. There’s a place, though, where you could take a lot of Category B jobs [more minor changes to a building] back out of this form of control into a more proportionate regime, potentially back with local authorities. Obviously that’ll be done carefully so we don’t get into a situation where we create new unintended consequences, but the answer, put simply, is yes. Because what that would then do is free up skilled resources inside the regulator [for] areas of much higher risk.”

It isn’t just windows either. The BSR’s is being prevented from clearing its massive backlog, in part, because it is stuck dealing with routine maintenance works like replacing fire doors and alarms.

This is bad for three reasons. It blocks flatowners from doing up their homes. It delays the construction of new homes leaving people trapped in overcrowded accommodation. And crucially, it makes us less safe because it makes replacing fire doors and alarms more expensive.

Chris’s windows are in a state, but for the time being he can afford to wait to fix them. With at least 35,000 homes held up by Building Safety Regulator delays, the Government can’t.