Planning departments have a scary reputation in some towns. We all know stories, hopefully told to us rather than experienced by us, of planners refusing to grant permission to applications that sound reasonable and straightforward. So far, at Inside Out Oxford, we have not had an application refused. We don’t know for sure, but we like to think that’s because our designs are attractive and sympathetic to the surroundings, and because our project descriptions use wording that helps to sell the intent. Garden rooms used as annexes for a retired parent, for example, help families to stay together affordably and take the load for care off the state. You can pitch that persuasively and appealingly, or miss a trick and not even mention it. And remember, garden rooms are here to stay, post-pandemic. Hybrid working both from home and in the office is much more likely to be the norm for most businesses that can afford that, and nobody likes to set up office in the kitchen, or a child’s bedroom.

Well yes, but i am also hoping someone might have an experience they can share with me.

Flats unfortunately don’t have the same Permitted Development Rights (PDR) as houses so i think you’d need planning (the same for houses if it’s listed, in a Conservation Area or if there’s an Article 4 Direction in place which removes PDR). I assume there are also a lot of Conservation Areas in Camden anyway! are you freehold or leasehold as you may also need permission from them?

No but you cannot use his wall. That would be a tresspass. You need to build all 4 walls on your property. If it's the wall to his house then party wall regs might apply depending on the depth of your foundations.