You can, but in Holloway an extension usually starts with a conversation with the council. There are hundreds of listed buildings in and around the area. Nine conservation areas reach into it. The ground beneath it swells and shrinks with the weather. Plenty of neighbours apply to alter their homes anyway, and none of them skipped the paperwork.
Historic England lists 242 buildings in and around Holloway: one at Grade I, ten at Grade II* and 231 at Grade II. Listing covers the inside of a building as well as the front, so any change to a listed home's character, from the windows to the walls, needs listed building consent on top of any planning permission. Working on a listed building without that consent is a criminal offence, so it is not a risk worth taking to save a few weeks.
Nine conservation areas fall in and around Holloway, among them the Bartholomew Estate, Jeffrey's Street, Camden Broadway and Brownswood. Inside one, the council looks harder at anything visible from the street, including rooflines, front boundaries and materials, and demolishing a building generally needs consent as well. The terrace you have in mind may look like a blank canvas, but the council will have views on the colour of the frame.
People in Holloway do alter their homes. The council's register logged 1,028 planning applications between April 2024 and July 2026, and 410 in the most recent twelve months. That says nothing about how many were approved, only that the neighbours keep trying. Islington's register is searchable by street, and reading what the houses next to yours applied for, and what came back, is the cheapest planning advice a buyer will get. "Scope to extend, subject to planning" in a set of particulars does most of its work in those last three words.
The British Geological Survey puts the ground across the whole of Holloway in class 3 for shrink-swell, the clay that expands when wet and contracts as it dries. Every part of the area reaches that class. Clay like this is very common in London and is no reason to give up on an extension. It does mean foundations may have to go deeper than a builder's first quote assumes, and a structural engineer should see the plans before any digging starts. Mature trees near the line of the new wall matter more on this ground than elsewhere.
Eight listed buildings in and around Holloway are on Historic England's Heritage at Risk register. That means those buildings are in poor condition. For their owners, repairs may qualify for grant support and will certainly draw close attention from conservation officers. For the buyer of an ordinary flat or terrace on the next street, the register has no direct consequence.
Day to day, the house you buy decides how much freedom you have. A terraced house outside a conservation area and not listed can usually rely on permitted development for modest work at the back, though even that should be confirmed with the council in writing. Most homes here are flats, and a flat needs planning permission for more of its changes and the freeholder's consent on top. Listed buildings and conservation-area frontages need consent for almost anything visible.
Extending in Holloway is possible, but it is tightly controlled. Owners do get work done here when they bring the drawings, the engineer and plenty of patience to the council.