No permitted development rules changed in 2026. That single fact is worth stating plainly, because the internet is full of guides quoting bigger extension limits that were proposed, widely reported, and never actually became law. Anyone relying on permitted development needs the rules as they stand, not the ones that were floated.
Here is what is real in 2026, what the proposals would have changed, and how it all plays out in Harrow specifically.
The Proposals That Never Landed
In 2024 the government consulted on relaxing permitted development. The headline ideas were increasing single storey rear depths, removing the 50 per cent curtilage rule, and loosening loft conversion limits.
Those proposals were widely written up, and a lot of that coverage is still online, presented as if it were current. It is not. The consultation closed in April 2024 and no government response has followed, so nothing from it has taken effect.
There is also a private member’s bill in the Lords proposing bigger extensions, but private member’s bills rarely become law, and this one has not.
The practical takeaway is simple. If a guide tells you a semi detached house can go 4 metres or a detached house 5 metres under standard permitted development, it is quoting a proposal. The real figures are below.
What Standard Permitted Development Actually Allows
These are the rules in force in 2026, set out in the General Permitted Development Order, Schedule 2, Part 1.
For a single storey rear extension, the standard limit is 3 metres beyond the original rear wall for a semi detached or terraced house, and 4 metres for a detached house.
Maximum height is 4 metres overall, dropping to 3 metres at the eaves where any part sits within 2 metres of a boundary.
For a loft conversion under Class B, the allowance is 40 cubic metres of additional roof space for a terraced house and 50 cubic metres for a semi detached or detached one. Volume is cumulative, so any previous roof addition counts.
For outbuildings under Class E, a garden building must be single storey and cannot be used as sleeping accommodation. A garden office is fine. A garden room with a bed in it is not, and that is the rule most often broken.
The Larger Home Extension Scheme
The one route to genuinely bigger depths is real, but it is not automatic.
The Larger Home Extension Scheme lets a single storey rear extension reach 6 metres for a semi detached or terraced house, and 8 metres for a detached one. The catch is that it requires the prior approval process, which includes a neighbour consultation period. If a neighbour objects, the council assesses the impact and can refuse.
So it is permitted development with a step in front of it, not a free extension. You must complete prior approval before starting work, or the extension is unlawful regardless of its size.
The 50 Per Cent Rule That Catches People
The limit that trips up the most ambitious projects is the curtilage rule.
All extensions and outbuildings combined cannot cover more than 50 per cent of the land around the original house, as it stood in 1948 or as first built. A big rear extension plus a garden room plus a side infill adds up fast, and once the combined coverage passes half the original plot, the later additions lose permitted development.
That word “original” matters. Previous owners’ work counts against your allowance, so a house that has already been extended may have far less headroom than the plot suggests.
Where London Diverges
Permitted development is a national framework, but London strips it back in ways the rest of England does not.
Conservation areas remove side extensions from permitted development entirely and bring roof alterations, cladding and window changes into the planning system. Article 4 directions go further, letting a council withdraw permitted development for a specific area or property type, sometimes street by street.
Flats and maisonettes have no permitted development rights for extensions at all, which matters in a city with so many of them.
So in London the national rules are only the starting point. The address level position is what actually decides your route.
What This Means in Harrow
Harrow is a useful example, because it is a borough of two halves for permitted development purposes.
Much of Harrow is unconstrained suburban housing, a lot of it interwar semis on generous plots with hipped roofs. On that stock, permitted development does real work. A rear extension within the 3 metre limit, a hip to gable loft conversion within the Class B volume, and a garden room under Class E can often all proceed without a planning application, and the wide plots mean the 50 per cent rule bites later than on a tight terrace.
But Harrow also operates conservation areas and Article 4 directions in specific locations, and inside those the freedoms narrow sharply. Side extensions stop being permitted development, and alterations visible from the street come back under control.
That split is why two near identical Harrow semis on different streets can sit in completely different positions. The check that matters is not what the house looks like, it is what designation applies to that exact address. Confirming that first is the single most useful thing architects harrow homeowners can do before committing to a design.
Before You Rely on Permitted Development
Three checks settle most of it, and all are free.
Confirm your property is not in a conservation area and carries no Article 4 direction, checked on your exact address rather than your street.
Work out what the original house was, because the 3 metre, 40 cubic metre and 50 per cent limits are all measured against it, not against the house as it stands today.
Then, if the work qualifies, apply for a Lawful Development Certificate. It is not compulsory, but it proves the work was permitted development, and a buyer’s solicitor will ask for that proof when you sell.
Rely on a proposal that never became law and you risk building something unlawful. Rely on the current rules, checked against your own address, and permitted development remains the fastest and cheapest route to more space there is.
