Sometimes, eventually, and nearly always with a council officer reading over your architect's shoulder. Islington has about 1,100 listed buildings. There are two dozen conservation areas in and around it. Most homes are flats, so most owners have nothing to extend in the first place. Building work is common here, and so is the paperwork that comes with it.
Historic England lists 1,100 buildings in and around Islington. Of those, 14 are Grade I, 34 are Grade II* and 1,052 are Grade II. Listing protects the whole building, inside as well as outside, so changes that would be routine elsewhere, such as replacing windows, moving a staircase or taking out a fireplace, can need listed building consent on top of planning permission. Carrying out work without that consent is a criminal offence. A Grade II house is the most common kind here, and it still comes with rules about the glazing bars.
On top of the listed buildings, 24 conservation areas fall in and around Islington, some of them crossing into neighbouring boroughs, such as De Beauvoir Town's. Inside a conservation area the council takes an interest in the street scene as a whole. Changes to roofs, front walls, windows and side extensions get a closer look, and the council can also remove some of the rights to alter a home without permission. On a terraced street, the extra floor you have in mind is effectively shared with every neighbour who wants the roofline to stay as it is.
People still try. Islington recorded 3,443 planning applications across the area between March 2024 and July 2026, and 1,495 of them came in the most recent twelve months. That is a lot of loft conversions, rear extensions, window replacements and changes of use going through the system, and every one of them took someone's time and money.
The British Geological Survey rates the ground at shrink-swell class 3 at its worst, and every part of the area reaches that class. That means clay soil that swells when wet and shrinks in a dry summer, which matters a great deal to a new extension sitting next to a house built on Victorian or Georgian foundations. Expect a structural engineer to specify the depth of the new foundations, and expect those foundations to be deeper than the builder's first quote assumed. It adds cost and design work, and it is no reason to panic.
Historic England's Heritage at Risk register lists 26 buildings in the area. These are buildings in poor condition whose repair can attract grant support and always attracts close scrutiny. For the buyer of an ordinary house next door, they are a sign of how seriously old buildings are taken here, not a hazard.
What this means in practice depends on what you buy. The council tax list shows about 86% of Islington's homes are flats, and flats do not get the permitted development rights that let many house owners build small extensions without a planning application. A house outside the conservation areas, and not listed, has the most freedom here. A listed house inside one has the least. If the house has already been altered, ask the seller for the consents covering that work, because unauthorised changes to a listed building become the new owner's problem on completion.
Altering a home in Islington means applying, waiting and sometimes being refused, and that is about as hard as it gets for a buyer with plans. Buy the house you can live with as it stands.