Double Storey Extensions: The Two Rules That Decide Yours

From our Fieldway Crescent project — Norbury, SW16

A rear extension of more than one storey is permitted development only if it passes two tests, not one: no more than 3 metres beyond the rear wall of the original house, and no closer than 7 metres to the boundary opposite that wall. Both, or neither. And on article 2(3) land — which includes every conservation area — Class A permits no rear extension of more than one storey at all. Planning decides whether you may build it. The foundation trench decides what it costs.

A single-storey rear extension pushes the ground floor into the garden and stops. A double storey carries the same footprint up to first-floor level, so what would have been a flat roof becomes a room. That is the point of it: a single-storey extension gives you a better kitchen, but it does not change how many people can sleep in the house.

It is also why the planning route is harder. A two-storey mass at the back of a terrace casts a longer shadow than a single storey, and that is the objection neighbours raise first.

The 3 metre and 7 metre rules that decide your planning route

Permitted development for house extensions sits in Class A, Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015. A single-storey rear extension faces one depth test. An extension of more than one storey faces two, and has to pass both.

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.

The two clauses that end it for most London houses

First, the 7 metre boundary rule. A 12 metre garden leaves 9 metres clear after a 3 metre extension, which passes. An 8 metre garden — ordinary in Fulham, Hackney and Tooting — leaves 5, and Class A is gone whatever depth you choose. Nothing in the design fixes it, because the test is measured from the boundary rather than from the house.

Second, article 2(3) land, on which paragraph A.2 permits no rear extension of more than one storey at all. Large parts of Hammersmith and Fulham, Wandsworth, Kensington and Chelsea, Camden and Islington sit inside a conservation area. There it is a full planning application from day one, and the programme has to carry a determination period.

Where a scheme does sit inside Class A, apply for a Lawful Development Certificate anyway. A buyer's solicitor will ask for it years later, when the evidence is much harder to assemble. Our post on permitted development rules for London extensions covers what that application contains.

Foundations — where a double storey parts company with a single storey

A second storey roughly doubles the load arriving at the foundation. The trench does not double in depth, but depth stops being governed by frost and starts being governed by how the ground behaves — and in London the ground is clay.

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.

Two qualifications sit inside that clay clause and both cost money. Shrinkable clay means a Modified Plasticity Index of 10% or more. And those depths are floors, not answers — the same clause says to increase them to transfer the loading onto satisfactory ground, or where there are trees nearby.

Trees move the number here more than anything else. A mature tree within influencing distance of a clay foundation drives the trench well past 1.0 m, and it need not be your tree. One removed a few years ago is worse than one still standing, because the clay is still recovering. The mechanics are in foundation depth and why it changes on site.

Why the party wall count changes too

A deeper trench brings a second section of the Party Wall etc. Act 1996 into play. Raising the shared wall is work to a party structure and needs a notice at least two months ahead under sections 2 and 3. Section 6 covers excavation separately: within 3 metres of a neighbouring structure and below the level of its foundations, or within 6 metres and cutting a 45° line beneath them, on at least one month's notice.

A single-storey extension on shallow footings often engages only the first. A double storey founded on a metre of clay engages both, on both sides of a terrace. What that changes is not only cost but sequence — the notices are served at different times and run different clocks.

What the building regulations require once planning is settled

Planning decides whether you may build it. Building regulations decide what it has to be.

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; the Approved Document shows one way of meeting them. Your building control body has the final word on your specific building.

What changes on fire when you add a floor

Adding a storey at the back of a two-storey house normally keeps every floor within 4.5 m of ground, so the new bedroom needs an escape window meeting paragraph 2.10 and nothing more.

The trap is a house that is already three storeys. Cross 4.5 m on any storey and a protected stairway is required through the whole house — a whole-house cost triggered by a rear extension. That belongs in the feasibility conversation, not in a variation six weeks into the build. We have written about what actually sits inside two quotes for the same job, and this is the classic line one of them has left out.

The five things that move the price

There is no double storey rate on this page. 2VP has delivered single-storey rear extensions and side returns rather than double storeys, so any figure here would be typed rather than billed. For your own house, the estimator prices it line by line from the same model we price real work with. What is worth understanding is how the price is built, because a double storey is not two single storeys stacked.

Sitting outside any build figure: the architect and the structural engineer, the party wall surveyor on each affected side, and the determination period, which is not a cost until you are renting somewhere else. Party wall fees have their own range and their own logic, set out in what a party wall agreement costs.

The one place a double storey genuinely saves money

Per square metre it should be cheaper than a single storey. The foundation, the ground works, the drain connection and the scaffold are bought once and carry two floors, so the rate falls even as the total rises.

That saving is conditional. It holds only if both floors are built as one project. Adding the upper floor in three years means stripping a roof you have paid for and re-erecting scaffolding around a finished house. If you want the upper floor at all, it is materially cheaper built now.

When a double storey extension is the wrong answer

Two of these are absolute and the rest are judgement.

The comparisons above have their own posts: which loft conversion type suits your house, the side return extension guide, and underpinning explained for London homeowners if movement is what is really on your mind.

Eight checks before you commission a drawing

1. 01Measure to the rear boundaryMark where the new rear wall would sit and measure to the boundary opposite it. Under 7 metres and Class A is gone, whatever the depth.
2. 02Check whether you are on article 2(3) landYour council's map answers it in a minute. In a conservation area, no rear extension of more than one storey is permitted development at all.
3. 03Establish where the ORIGINAL rear wall wasThe 3 metre limit runs from the original dwellinghouse, and paragraph A.1(ja) measures any total enlargement against the same limits. A previous owner may have spent the allowance.
4. 04Dig a trial hole against the existing rear wallBefore anyone prices the groundwork. It shows the footing you are tying into and the ground you are founding on — the two things that move the number most.
5. 05Map every tree within influencing distanceYours, the neighbours', and any removed in recent years. On clay these drive the trench past the Approved Document A minimums, and a removed tree is worse than a standing one.
6. 06Count the party wall notices, on both sidesRaising the shared wall engages sections 2 and 3 at two months. Excavating below the neighbour's footings engages section 6 at one month. On a mid-terrace, assume both, twice.
7. 07Check the escape route against 4.5 metresEvery storey within 4.5 m of ground and an escape window will do. Above it, price a protected stairway through the whole house before you commit.
8. 08Price the single storey alongside itSame drawing, same finish, same day. The upper floor should have to justify its cost in writing, against the alternative, before you sign for it.

The one line to take away

Measure the 7 metres to the rear boundary first. On a typical London garden that single dimension decides whether this is a certificate or a planning application — and it costs nothing to establish, unlike everything that follows it.

2VP is the builder, not a marketplace: one contract, one named project lead, and no introducer fee stacked on the build cost. On a job where the foundation depth is found rather than designed, the question of who is accountable when the ground disagrees with the drawing is not an abstract one.