You can, but Highbury will ask a lot of questions first. The area is full of listed buildings. Conservation areas cover parts of it. The ground is clay that moves. And most homes are flats, where an extension needs other people's agreement and a planning application, not just your own wish.
There are 173 listed buildings in and around Highbury: three at Grade I, seven at Grade II* and 163 at Grade II. Listing covers the whole building, inside and out, so changing almost anything in a listed home, from an internal wall to a staircase, needs listed building consent from the council on top of any planning permission. Work done without it is a criminal offence, not just a planning problem. Most of those 163 Grade II buildings are ordinary homes and terraces whose owners took on the protection when they bought.
Six conservation areas fall in and around Highbury, including Newington Green (North), De Beauvoir and Clissold Park. Inside a conservation area the council takes a closer interest in anything visible from the street, from roof extensions and dormers to demolishing a front wall, and some works that would be automatic elsewhere need an application. Find out which side of a conservation area's boundary a house falls on, because the edge often runs between two streets that look identical.
None of that stops people trying. Islington recorded 740 planning applications across Highbury between March 2024 and June 2026, 338 of them in the most recent twelve months. Plenty of owners are applying to change their homes. The neighbours read the planning notices pinned to lamp posts, and a street of period houses usually has a few residents who care a great deal about what goes on next door.
The ground matters too. The British Geological Survey puts all of Highbury in class 3 for shrink-swell, which means the clay underneath expands when wet and shrinks when dry. That doesn't stop an extension, but it does affect how deep the foundations need to go, especially near trees. A structural engineer should look at the ground before the architect starts drawing, so the clay is built into the design from the start.
Historic England lists four buildings in the area on its Heritage at Risk register. These are listed buildings in poor repair, and their owners face both close scrutiny and possible grant money. For someone buying an ordinary house nearby they make no difference, unless the house is one of the four.
In practice the rules split three ways. A house outside a conservation area and not listed has the normal householder permitted development rights, which cover some rear extensions and loft work without a full application. A house in a conservation area has fewer of those rights. A listed house needs consent for almost everything. Many homes here are flats, and flats do not get householder permitted development rights at all, so even a modest change needs permission, plus the freeholder's agreement under the lease. The question a buyer actually needs answered isn't "can it be extended?" but "which of those three is this house, and what does the lease say?"
The freedom to change a home in Highbury is limited, and for a listed house or a flat it is closer to supervised than free.