Replacement window- where to start?

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The window in this room, which is a first floor box room that I use as a dressing room, is single glazed, rattles and the timber frame is rotting.

It has just occurred to me that I could replace it with a nice double glazed picture window and it would be absolutely fine for it not to be timber as the window is at the back of the house and the only one on that wall.

I would like to make it longer, almost to floor level. The house exterior is basic brick.

Where on earth would I start? Do I go to a double glazing company, a builder, or what? Do I have to know what style or material I want before I choose the window company?

Any recommendations for good window people in London/SE?

Exterior view

I don’t think I would make the window larger, I would have thought that would be a huge unnecessary expense, it is a good enough size already for a small room. Looking at your second photo, I would get some repointing done, it looks like there is one rather large hole forming in one place under the window where the mortar has all come out, looks like other bits might need repointing too.

Thanks, but I’m not really motivated by cost saving, I want a lovely new window, not just a plastic replacement for what I already have. So to me, paying for the larger window is a “necessary expense”.

We’ve looked at the bricks before, they are more robust than you might think and it’s only the back. Might get it touched up when the window is being done but not doing the whole house.

Also that room feeds light all the way to the front of the house, the house is narrow but long, and a terrace so no side windows.

Start by looking for a local glass shop that installs FENSA compliant windows.

You need a builder first. They will be able to see how low it can go. Looks like you could make it approx 500mm deeper.
While they are at it, they’ll be able to advise about your dropped brickwork. Looks like a window or doorframe under your window has issues and may need a lintel putting in.

Its not a big job , ive done it a few times. You could reuse the cill ( which ive done before ) by just removing the desired amount of brickwork and just rebedding it down lower . 2 days for someone to do the brickwork inside and out then you would need to get the inside plaster done, but wait till the new window is in as more plaster might fall off when they take the window out.

Leave it just high enough to run the skirting under the window inside .
i like full length windows too .

If you want a nice wooden sash window with double glazing you need a joinery firm that has a large manufacturing workshop They will measure for the window, manufacture and fit it. It could be supplied ready painted and you will get a choice of glass, self cleaning, less heat loss etc.

If you want to make it bigger you’ll need a builder as well.
Are you in a conservation area or is the house listed? If the answer is yes to either then you need to ask the planning department

Looks like it's single skin construction there which could be a problem..

You need a builder first. We're about to do something similar in our kitchen which has timber Victorian sash windows, but we need to make the one facing the garden smaller as at the moment the lower end drops below the level of the worksurface.

Our builder didn't think it would be an issue and said his window people should be able to make us a double glazed sash window to fit the new space, I think in UPVC. He hasn't got back to us with his quote yet so I haven't got a name for you or been able to check finishes etc.

I'd also not worry too much about UPVC v wooden sashes - we used to live in a Victorian flat conversion and our upstairs neighbour replaced her windows with nice UPVC sash ones and they didn't stand out against our wooden ones at all.

We had a new door put in - brand new so needed the brick work cutting - and the window people did it all so id check first, before you end up trying to source a separate builder and window firm.

Thanks everyone for your helpful replies.

What do you mean?

Nope. Bog standard terrace.

No, I definitely don’t want an expensive wooden sash window, as I explained in my OP it occurred to me that I wasn’t limited to one of those in the same way as I would be if it were at the front of the house.

When you say single skin , you mean solid 9 inch wall and it wouldn’t be a problem. There is already a window there all the op wants to do is make the window longer.