Rarely, and never without permission. The City of London has 1,311 listed buildings. Nearly every home is a flat, so often there's no house to extend in the first place. The clay underneath is as fussy as the history on top. People do still try, and every attempt begins with an application form.
Historic England's list puts 1,311 listed buildings in the City: 119 at grade I, 132 at grade II* and 1,060 at grade II. Listing covers the inside of a building as well as its frontage, so changing a staircase, a fireplace or a window in a listed flat needs listed building consent from the City of London Corporation, and the grade I and II* buildings get the closest scrutiny. Whether your block is listed is a matter of public record, so look it up on Historic England's list and ask the freeholder what consents the building has needed in the past, since a leaseholder also needs the freeholder's agreement for any alteration.
There are 48 conservation areas in and around the City, from Leadenhall Market and Fleet Street to Smithfield and the Temples. Inside one, changes to the outside of a building that would pass without comment elsewhere can need consent, and demolition and work to trees come under extra control. Two buildings facing each other across a lane can fall under different sets of rules.
The surrounding area also holds 63 scheduled monuments, the strictest tier of protection, including sections of the Roman wall and the site of the Roman amphitheatre under Guildhall Yard. They matter mainly to anyone digging. Sink a basement near Guildhall Yard and you'll find the Romans were there first.
The planning register shows plenty of people still apply. There were 2,682 applications in the City between April 2024 and June 2026, and 1,263 in the most recent twelve months. Most of the Square Mile is offices, so many of those come from businesses rather than homeowners, but the volume shows the system is busy, not frozen.
The British Geological Survey puts the City's shrink-swell hazard at class 3, and every postcode in the area reaches that class. That means clay that expands when wet and shrinks when dry. It doesn't stop building work, but any new foundations will need to go deeper than usual and a structural engineer should sign them off.
Fourteen listed buildings in and around the City are on Historic England's Heritage at Risk register. Those are buildings in poor condition whose repair may attract grant money and will certainly attract scrutiny. They tell you how closely the Corporation and Historic England watch the City's buildings, and they pose no risk to the buyer of an ordinary flat nearby.
In practice, the householder permitted development rights that let people extend houses elsewhere without planning permission apply to houses, and about 96% of the City's homes are flats. For most buyers here, almost any change beyond redecorating goes through the Corporation, often Historic England and usually a freeholder as well.
For anyone hoping to extend or alter a home, the City is about as restrictive as England gets. If you buy here, buy a flat that already has the space you need, because adding more is a long process.