Sash Window replacement advice needed!

Hello. I am looking to replace the sash units in my sash windows with double glazed ones. I won’t be replacing the actual window frames - just the sash units inside.

I live in a flat in one of the Forest Hill conservation areas.

I was wondering if anyone has any experience of doing this and knows whether planning permission is required for replacing only the sash units (I will be replacing like for like with wooden sash units).

I have called Lewisham planning department, but all they have done is refer me to sections of their website on replacement windows that I’ve already read and don’t answer the question! They also suggested getting pre-planning advice from a planning officer, but that seems to be aimed more at larger projects than merely replacing windows. Ideally I don’t want to pay for that service unnecessarily.

Help please! Thankyou in advance!

Did you read this bit? Does it answer your question?.. probably not to the extent you want in order to get it done :man_shrugging:

Windows in Conservation Areas and on Locally Listed Buildings.
On elevations visible from public viewpoints (usually the front and sometimes the sides and rear of
buildings) we always expect replacement windows to be of the same material as the original
(usually timber) and exactly the same type and design. The majority of the borough’s historic
housing stock have traditional timber sliding sash or casement windows. Their replacement with
uPVC or aluminium windows will not be accepted.
We normally permit the change to double-glazing, although slim-profile double glazing is preferred
(slim-profile double-glazing has a slimmer cavity, and therefore thickness than conventional doubleglazing). On non-visible elevations (usually the rear of the building) we encourage the same, but will
sometimes permit the use of a non-traditional material (such as uPVC or aluminium) provided the
overall type of the window (its design, the glazing and opening pattern) are similar to the original.

1 Like

Thank you @Beige, this link is very helpful as I think it’s relatively new guidance, which I’ve been hoping for for many years.

Previously, they outlawed UPVC sliding sash windows in conservation areas but I noticed an Appeal case in North London a few years ago, which I believe must have caught the attention of other Borough Councils.

The thing, for me, is that the actual material that a window frame is made of wouldn’t be apparent from the pavement or road so it’s the general proportions that matter. Unfortunately, UPVC window fame sections tend to be slightly thicker than the proper old traditional timber but, that aside, the really important thing is to have a deeper bottom rail and run-through horns on the top sash as these 2 features really do help for a good overall appearance.

Apart from aesthetics, there’s the practicality factor, which leads on to cost. These days, tradesmen aren’t always allowed to work from a ladder so you might need to factor-in scaffolding when wooden frames need to be repainted every 6 years, which makes the slightly thicker, but maintenance-free, UPVC option worth a bit more than a second look.

As with most things, it’s normal that suppliers want to cash-in on people’s dilemmas, so if a timber sash is £1,000 most UPVC ‘heritage’ window suppliers would come in at a [supposedly] very reasonable £695 for instance. However, I’ve found a brilliant firm who charge under £400 for any window up to 2.1m high by 1.1m wide - and that’s with slide & tilt openers but you add on a bit for glazing bars. Pre-Covid, they were running at 10 days from order to delivery and all totally bang on time. Hopefully it’s not outside of posting rules for me to give the heads-up on this firm (who also supply timber windows but I can’t vouch for those) - Sash Windows – Colin's Sash Windows

This is great info @Anotherjohn and @Beige. We got quotes for double glazed timer sashes last year and the total cost to replace all the windows was eye-watering. I honestly think it is bonkers that we are encouraged to make our homes more energy efficient but then there are so many barriers/high costs to achieving that.

1 Like

Have you spoken to someone to see whether they can re-use your existing frames? I seem to recall when chatting to Sash Guy (details on the other thread that I know you’ve commented on) when he refurbished my windows, that it wasn’t always possible as the counter-weights required for the heavier double-glazed sash wouldn’t fit. Then again he managed to salvage and draught proof some old windows that I honestly thought were past it.


Hello! Yes, I actually spoke to the Sash Guy about it as I’d seen several recommendations for him on this forum! He was very helpful and I can definitely keep the existing frames - the confusion lies in whether I would still need planning permission to replace the single glazed sash units inside the frames with double glazed ones. He said I don’t but I think, because my flat falls in the conservation area, I do - but it is very unclear on the council’s website, so I am a bit baffled!

Thankyou so much! Yes I did read this - it’s just a little confusing as to whether you need planning permission if you’re only replacing the sash units inside the frames and not the actual frames themselves. I think, because I’m in a conservation area, I will still need planning permission to ‘upgrade’ to double glazing, but I can’t seem to get a conclusive answer from the council!

I would go out on a limb and say that you don’t need planning permission because there would be no material or discernable visual difference to the existing windows.
The outside-facing part of the sash box stays the same and, providing the Sash Man copies the framing material, you wouldn’t be able to tell that there are 2 pieces of glass instead on the one you have now.
Some sashes, if they’re thick enough, could be routed-out to make the glazing recess deeper to take a thin double glazed unit.

Also, I the hypothetical situation that after fitting one hears from the council that planning permission should have been sought one could apply retrospectively. If the council planning website shows permission in the locality having been granted for similar windows in the past then a precedent would exist.

Planning can be a pain in the rear.
When applying for a big 2-storey extension with a smaller single-storey one to the side at my daughter’s house, the planning officer told me categorically that I’d need to make 2 separate applications in order for it to comply with their (Bromley’s) planning policies. I argued that it was compliant and told him that he was just being pedantic. Anyway, I had to go along with his nonsense and apply for the 2-storey bit first - but I built the single storey part anyway. Being pedantic, 2 years afterwards he initiated enorcement proceedings against me for the unauthorised development and I found a recent successful Appeal on a house just around the corner for exactly what I’d originally proposed, which meant that formalising it was pretty much a rubber stamping exersize after much ado about nothing.
That’s not my only run-in with planners where I’ve proved them to be completely wrong; and I just can’t understand why (in my experience) they always seem to be so unnecessarily obstructive.
Anyway, in this case, it’s effectively an extra layer of glass in a window that’s potentially being legislated for and it makes me want to scream.


I once paid the £80 for a pre planning consultantation. I couldn’t believe what I was told there so I ignored the advice and applied for a lawful development certificate anyway - it was granted. I paid £80 for some bad advice and some stress.

The problem with not having planning permission might come about when you try and sell the property. I doubt the council are diligent or care enough to issue an enforcement notice over something like that. We live in a square of listed buildings in a conservation area. Many have noncompliant uPVC windows.

The only time they’ve ‘come for them’ is when they’ve applied for other, unrelated planning changes and the inspector has noticed upon dealing with that application.

That said that’s much more of a flout than replacing ‘like for like’ and I reckon you’d be fine doing that. But be more careful if the conversation area has an Article 4 direction on it. This is a much stricter clause, but the majority don’t.

1 Like

I would also argue that changing or adapting the existing sashes, as opposed to the whole window, providing they’re like-for-like (except for the double-glazing), shouldn’t fall within the scope of a full-blown, pain in the bum, waste of everyone’s time, waste of your money (£206 planning application fee), waste of local taxpayers’ money (for administering the planning process) planning application. (They don’t call me Victor Meldrew for nothing!).

1 Like

My builder friend tells me that you can fit a single pane of glass that is manufactured to act like double-glazing. He says the single sheet of glass is hollow, which helps the heat insulation. Presumably, it should also reduce the weight.

1 Like

That would be a game changer!
It would be great if you could get more info on this please.
(So far I’ve found - About MONO: single-glazing system for period properties | Histoglass )

1 Like

Thanks so much to everyone for your help and advice. I’ve finally received confirmation from Lewisham Council that replacing the sash units inside the existing frames is just classed as maintenance / repair and does not require planning permission.

1 Like