I can’t think of a solution to my extension issue (WITH DIAGRAM!)

We don’t really want to extend the full width as it’s too close to the neighbors boundary.

It's not clear why you don't want to extend full width. Looks like you're building up to the boundary on one side, so why not the other?

We have a full width extension across the back of our house and I love it. We did this for the very reason you're describing - we didn't want to create a closed in corner. We have a seating area on one side which means we can see the whole garden.

The size of your extension looks about the same size as your problem garden area on the diagram- is it not worth thinking about just making the extension wider to give you usable space?

How much boundary are you required to have between the edges of yours and neighbours property?

You could then fill the red line area with plants so the view from the window is attractive.

We do have a similar area in our garden as a result of our extension and planning rules. Ours is north facing, so I’ve made it into a quiet shady garden. Because it’s more or less finished, I sit in there when feeling overwhelmed while working on the rest of the garden !

Put steel posts in next to your neighbours boundary. Extend the roof of your extension only to these steel posts creating a covered patio area. You can put bifolds of sliding doors out to it and have heaters wired in the ceiling. Meaning you can sit out in summer and winter. We have done this on a few properties now and it really works and opens up the inside outside space.

Was going to suggest this.

If you put the door to the garden entering into this area then it will naturally get used more.

Tbh op. It's a pretty large area and I don't think it will feel as cramped as you think. Lots of people have living rooms of a similar size.

There are lots of ways to soften/push back the brick walls. One trick is to hide the horizontal lines of the walls particularly at the floor level. You can built narrow ground beds or trough planters on some of the boundary. It's most effective to use a variety of heights and textures so build or purchase a bench/sofa that will face the garden. Leave a 400-600mm gap behind it so you can grow tall plants against the wall. Then you can scatter pots down the walls either side. The key is to use high volume planting with plenty of height.

Ornamental bamboo, grasses, vertical trending planted like salvia, climbing plants etc.

There is lovely decorative climbing wires you can install yourself that is more subtle and inexpensive then trellis for climbers.

In the gaps you could install lovely metal wall art pieces for interest.

I’d take the extension further across unless you haven’t any other outside seating area. We’ve got a similar space, but shady, which is handy for the bbq and I’ve filled with plants, acers mostly, in huge pots, that bring colour into the kitchen. We never sit there. But it’s a bit not one thing or the other as not big enough to sit in ….but a great view from the sink.

Sure, if you hate your neighbours!

I tried to get it to raise the height of the back wall but it wasn't playing ball, but you get the gist. I don't know how big your window is, but this looks ok.

You need to check planning regs. Your diagram has your extension going pretty much to the boundary on the other side but in many parts of the UK this is not allowed. Most areas insist that extensions are at least 1m from the boundary with a neighbour.

On the square area, I would put a fountain there and make a feature of it with some nice plants.

I live in a terrace where lots of us have extensions on the back. Ours is an almost full width one with patio doors across the full width and it’s south facing so gets lots of light. When we bought the house (with the extension) the kitchen was in the extension and the old kitchen became a middle room with very little light, we swapped things over and put the kitchen in the middle and the lounge area in the extension (it’s all one big room).
The other houses have extensions like you’re suggesting. They have narrower patio doors where the original back door was and the kitchen in the extension but more like galley style. Our neighbours all say they wish they had a full width extension.

There aren't any measurements, so is it actually worth it? For the cost, how much extra space will you actually get?

I would use it as a covered space to sit outside. But I shun the sun and hate having a south facing garden.

If the rest of the grey is seating area I'd just focus on filling it with greenery - it sounds like a lovely place for a mini jungle garden.

Like this https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%2Fid%2FOIP.bVihDQciHmQa9tCtZK5nqQHaJQ%3Fpid%3DApi&f=1&ipt=7a2bd944db5e60e29680f6da378ad08d21eb7987ad0ca7d38f7ac983d78837df

Not mine, but I’ve always loved this little area, so something like this? There is a door at the end leading to another part of the garden.

The entirely non-sensitive photo is under review. It’ll appear eventually, I hope.

Perfect space for a hot tub!

It would be protected from the wind so great place to sit when windy. I'd decorate the walls with greenery and make it a cosy seating area.

Make it a secret garden. Some screening, pretty plants climbing the walls, comfy seating, pretty lights. Lovely decor / mural on the brickwork.

We have a similar space. Ours is probably 4m x 3m (this opens to the grasses garden). We have laurels between us and the neighbours. A washing line that then be retracted when we are sitting out. We used to have outdoor wall art too but DH took it down. I loved it. He didn't.

Being enclosed means less wind in the summer for BBQs. You can make lovely with plants and lights.

Is the rest of the grey squiggle not patio too?

Sorry irrelevant. Didn’t read properly

Could you put patio doors on the purple and blue lines adjacent to the red line? We have a similar set up (but only with patio doors on your purple line equivalent), also with a south facing garden and it works well.

I'd put an awning over it, maybe glass roof and make a little conservatory/greenhouse? You could dry clothes there in the Winter, and grow plants in Spring, and put a little bistro chair and table in it for the escape room - only one person at a time, i.e. YOU!

I'm going to do this, when I get the funds but my garage where I'd put the lean to greenhouse is at the end of the garden (old Vic terrace), so I'd have to walk the length of the garden to escape ha ha, but still.....

if it's big enough, I'd create a little bistro area. Be nice to sit and have breakfast, or a drink of an evening. As long as you add some greenery and lights it could look quite charming.
image uploading.....

You own 2 of the 3 walls meaning you can attach things to them. That means planters with trellis, or you can use horizontal slim wooden battens like the very modern fences, plants in pots. You can buy mature instant hedging to hide anything you want.

Just google courtyard gardens to see what people manage to achieve with seating, planting etc.