-
Planning: Next door is about as good a precedent you can have for planning assuming your design doesn't look to reinvent the wheel too much!
Getting permission for a dormer plus extra room on the back should be a formality. I'm chancing my arm a bit with the addition of a small roof terrace (suitably set back from the rear wall) but my understanding is Harringey have recently started allowing them for flats. Permission was granted recently for one a couple of streets away almost identical to what I'm after.
Planning: Next door is about as good a precedent you can have for planning assuming your design doesn't look to reinvent the wheel too much!
Leaseholder Consent: You mention an independent freehold owner. Have you looked into whether you need to apply for leaseholder consent for your works and what this includes?
Party Wall: It is worth noting that your neighbour(s) can't stop the works from happening but they can be difficult, and at least you know where you are likely to stand with your downstairs neighbour and to factor additional moneys for this process. The party wall process does include rights for access which will include scaffolding etc.