Replacement windows in a leasehold flat?

You will need permission from the leaseholder for which you'll pay a fee but I would first ask if they are planning to upgrade the windows & serve a section 20 in the near future.

2 years ago I told my freeholder that I needed to replace my double glazed windows. My son was intending to do the work. I sent pictures of the ordered windows from the manufacturer and the freeholder was happy with that. They did not charge anything. In another flat, 16 years ago I did not get permission I just went ahead and did it. That probably would not be a good idea today. Too many rules and regulations.

Is this a purpose-built block?
When was it built (or converted if not PB)?
How many flats?
Do the UPVC windows appear to be original?
Do any other flats have replacement windows (ie. is there a precedent)?
The lease does not appear to be very well worded if it makes no reference to windows (ie. including the frames) only to glass. In my non-expert opinion this is ambiguous as it is impractical that the responsibility for wiindow frames is split external/internal.
I would be interested to read comments from those in the legal profession.

As is frequently advised, the lease dictates and should be your first port of call, which you appear to have done. Leasehold is difficult to generalise and its not uncommon to have different lease terms in the same block.

In olden days, when some builders built brickwork directly off timber frames, windows were very much a structural element; nowadays, less so. So, unless your lease is explicit, it's a grey area.

Many (but not all!) leases include window frames in the lessors responsibilities, with the glazing down to the lessee. Again, your lease will dictate.

Even if your responsibility, you will still need the freeholders permission if you need to erect scaffolding, say, or otherwise encroach on common parts to do the job.

FWIW, I'd contact the freeholder/managing agent for their understanding of the matter and look for a solution that benefits you both.

I have a flat in a 1970s built block ( share of freehold). The single glazed wooden windows were shot or expensive to keep repairing or painting. When i asked about replacing them 7 years ago I was told to just go ahead. When I asked 3 years ago I was told to check with the council and also submit plans and ensure FENSA compliance to the management company. Having seen that half the windows in the block had been replaced by UPVC double glazing anyway I decided to just go ahead as scaffolding and trees that were in situ anyway would hide the change. It's one of those. I know people who jump through all the hoops and get endless red tape and delay when they 'do the right thing'. I took a risk I know but fitted proper FENSA approved windows in keeping with the block. I felt mine were much better and in keeping than some in the block. I believe the council would have insisted on keeping the rubbish wooden ones despite the fact that 1/2 the block was now UPVC anyway.
In the name of progress.....

The starting point here is RTBL. Read The Bl**dy Lease ... to clarify where the flat stops and the rest of the building begins. You need to the words which describe the Demised Property.

From what you have said about the Reserved Property (that which is not demised to any flat), the glass in the window frames may be demised ... but the window frames are not demised.

Before you do anything, speak with the freeholder and/or managing agent (not letting agent) and get confirmation in writing of their view/position.

Reply to the comment left by yl2006 at 21/12/2022 - 11:16
Most windows are done from the inside so probably no need to encroach on common areas or use scaffolding

Reply to the comment left by Crossed_Swords at 21/12/2022 - 13:00
Don't see how. Even if it's a brick building (ie. no external render to break-off) and the window goes in perfectly, there will still be an external gap between frame and wall to fill.

Reply to the comment left by geester24 at 21/12/2022 - 12:51
Having to completely renovate a rental to live in for a couple of years, I faced the same issue with rapacious freeholders - they wanted money to grant permission - and then have to obtain planning permission from the council - yet more money down the khazi - I decided to go ahead without both. The replacements were completely in keeping with several other flats' windows and if either the council or freeholder want to make an issue out of it, good luck proving it was me wot done it. Bunch of robbers.

Reply to the comment left by Ian Cognito at 21/12/2022 - 13:25
Just done it for a whole block - all done from inside each property. Don't take my word for it - ask your local window company