Welcome neighbours - we're also in Streatham. So, remember that Planning and Building Regs are two different things. Firstly, Planning. Permitted Development rules do not apply to flats, so things that can normally be done to houses pretty informally (loft conversions and extensions within certain maximum sizes are examples) need a full planning application for a flat. Look at the planning portal and check what you need to apply for. Lambeth is annoying as you have to do a pre-planning application if you want an opinion (they got rid of the duty planning officer advice a while back to cut costs) - a bit of a scam really, you would be better off hiring a proper planning consultant/architect used to Lambeth and getting your application right first time. We did everything under Permitted Development (house), so did not have to go down that route. Building Regulations then need to be complied with when doing the work allowed under Planning. Rather than organise site visits/inspections with the council, who are notoriously hard to get in touch with, we used a private company, London Building Control, who were excellent. It costs a bit more, but you don't spend your whole life chasing council employees who are unmotivated and useless (in Lambeth's case, anyway). Generally, internal changes (removing walls and using steels etc) do not need planning permission of any sort, but are definitely a Building Regulations issue: and you will need to show proper structural engineering drawings with calculations and steel specs etc. Good luck!

Flip the stairs 90 degrees so they are on right hand wall (ie you walk through door and go straight ahead up the stairs) Turn the remainder of the living room into a small snug, with a door to the rest of the ground floor. Extend into your side return and create open plan kitchen/dining+ seating area if there is room. Open plan area would replace what is currently the sitting room, kitchen and side return. Keep the utility you want at the end. Upstairs - front bedroom loses a little width because of new stairs but can be widened the other way, creating a better proportioned room. Small landing with access to other bedroom and bathroom. If you want you could build up your side return extension to a 2 storey to create more room for the bedrooms upstairs. Just an idea, hope this is helpful! Is the house terraced/semi/detached?

Hi One Plan...I know, and I agree that it seems strange but here’s the thought process. I’m 52, recently single and so it might well be my long term home and what’s important right now and what I can afford. I could extend the mortgage to do a full side return but that would make my monthly payments strangling as I also need to think about how long I’m going to keep working. What’s important is having entertainment space - which I can’t create currently so this was my next. Thought ( or move to something already suited to me but with stamp duty etc that works out just as expensive unless I move much further out and I’ve pretty much ruled that out. It’s a amazing flat loads of storage ( basement) and huge garden and prime location so I’m trying to make it perfect for me ( not necessarily resale atm).

Is there a reason for a second bedroom? I ask as given your budget and the current increasing costs of materials might put a full extension out of budget. Given that,  I also note that your current combined kitchen lounge area is larger than the bedroom, so it may be more within budget to think about the layout of these 2 rooms. I see that the current bedroom has an attractive outside aspect. If the kitchen is reconfigured you might be able to achieve this from this room with double doors located next to the gate. If it is possible to knock through between kitchen and lounge you could achieve an open plan or almost open plan in this area with the outside aspect. Some food for thought.........