Upward Extensions London 2026: Adding Storeys Under Permitted Development
Adding one or two storeys on top of an existing house under Class AA permitted development has been possible since August 2020. Here is what the rules actually say, what it costs in London, and why the structural reality is considerably more complex than it sounds.
Quick Answer
Upward extensions cost £120k–£320k+ inc VAT in London. Adding one storey above a terraced house runs £120k–£200k; two storeys above a detached house runs £180k–£320k. Class AA permitted development allows upward extensions without full planning permission, but prior approval is always required. Structural costs are the dominant variable.
£120k–£200k
1 storey (terraced)
£180k–£320k
2 storeys (detached)
£4,500–£7,000
Cost per m²
Check your specific property constraints
Free Property CheckWhat Is an Upward Extension?
An upward extension adds one or two new storeys on top of an existing building. In planning terms, these are distinct from a loft conversion: a loft conversion works within the existing roof space, while an upward extension removes the roof entirely and constructs new floors above the existing top storey.
The Class AA and Class AB permitted development rights, introduced in August 2020 under the Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2020, were a significant policy change. For the first time, householders and flat-owners could build upward without needing full planning permission — provided they met the conditions and obtained prior approval.
Class AA covers houses (detached, semi-detached, and terraced). Class AB covers purpose-built blocks of flats. Both rights require prior approval from the local planning authority before work begins. Neither is an automatic right.
Upward extension vs loft conversion: the key difference
Loft conversion
- Works within the existing roof void
- Roof structure largely retained
- Lower structural intervention
- Class B, C permitted development (different rules)
- Typical cost: £60k–£120k
Upward extension
- Whole roof removed, new floors built on top
- Major structural intervention throughout
- Existing foundations must carry new load
- Class AA or AB permitted development
- Typical cost: £120k–£320k+
Class AA: Houses
Class AA applies to engineering operations and building works on a dwellinghouse to create new storeys above the existing top storey. It covers detached houses, semi-detached houses, and terraced houses — but the rules differ depending on house type.
Class AA rules at a glance
Detached houses
- Maximum 2 additional storeys
- Must not exceed 18m above ground level
- Must not exceed the original roof height plus 7m
Semi-detached and terraced houses
- Maximum 1 additional storey
- Must not exceed 18m above ground level
- Must not exceed the original roof height plus 3.5m
Conditions applying to all Class AA works
- The house must have been built on or before 28 October 2018
- No new extension beyond the existing exterior walls at the top of the existing house
- New storeys must sit within the same footprint as the existing house
- Prior approval is always required — there is no automatic PD for upward extensions
- No windows on the side elevations (to protect neighbour privacy)
In practice, the 3.5m height limit for terraced and semi-detached houses means one new storey of approximately 2.4–2.7m ceiling height is achievable, with some allowance for floor structure and roof covering. For detached houses, the 7m allowance comfortably accommodates two full new storeys.
Class AB: Purpose-Built Blocks of Flats
Class AB permits upward extensions on purpose-built blocks of flats. The key conditions are:
Class AB is particularly relevant in London where purpose-built post-war flat blocks are common. However, executing an upward extension on a leasehold block requires the agreement of all freeholders and leaseholders — a legal and governance challenge that often makes Class AB impractical despite the planning route being available.
Prior Approval: What Councils Assess
Unlike most permitted development rights where works can proceed without any application, Class AA and AB always require prior approval. This is a mandatory check before work begins.
The prior approval application must include drawings, a structural report, and a statement confirming the works fall within Class AA or AB. The council then assesses against a defined list of matters:
What the council considers for Class AA prior approval
The prior approval process
Application fee: £120 per storey or £234 for the whole application. Drawings and structural report required at this stage.
The council must consult adjoining owners and occupiers. Neighbours have 21 days to make representations. The council must take any representations into account.
The council has 8 weeks to approve or refuse. If no decision is issued within 8 weeks, you can appeal or proceed — but verify the legal position with your architect before doing so. Approval may come with conditions.
Prior approval does not replace building regulations. Full plans submission to building control is required before works begin on site.
Refusals under prior approval are less common than under full planning permission, but they do happen — particularly where the council considers the new storey will significantly harm the streetscene or where neighbour objections about overlooking are compelling. Unlike a full planning refusal, the scope for the council to refuse is limited to the defined list of matters.
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What Class AA and AB Do Not Cover
Class AA and AB rights are excluded from a significant range of property types. In London, these exclusions affect a large proportion of the housing stock:
Listed buildings
Any property that is listed (Grade I, II*, or II) cannot use Class AA or AB. Full planning permission and listed building consent are required for any upward extension.
Conservation areas
Class AA and AB do not apply in conservation areas. London has over 1,000 conservation areas. This excludes a very large proportion of inner-London houses from these rights.
Article 4 directions
If the local planning authority has issued an Article 4 direction removing Class AA or AB rights in a specific area, full planning permission is required. Several London boroughs have used Article 4 directions to restrict these rights.
Within 1km of a licensed aerodrome
Properties within 1km of a licensed aerodrome (not just major airports — this includes smaller aerodromes and heliports) require consultation with the Civil Aviation Authority. The rights can still apply but approval may be conditioned or refused on aviation safety grounds.
Sites of Special Scientific Interest (SSSI)
Properties within or immediately adjacent to an SSSI are excluded from Class AA and AB rights.
Houses built after 28 October 2018 (Class AA)
Class AA only applies to houses built on or before 28 October 2018. Newer builds must use full planning permission for upward extensions.
Not sure if your property is in a conservation area or subject to an Article 4 direction? Our AI checks your address against planning data instantly.
Check now — it's free →2026 Costs for Upward Extensions in London
All figures below include VAT at 20%. Upward extensions are significantly more expensive per square metre than conventional horizontal extensions. The difference is structural: the entire roof must come off, temporary support must be installed, the structure assessed and reinforced, and then new floor and roof structures built. These costs apply regardless of the size of the new space.
| Scope | Construction | All-in |
|---|---|---|
| 1 storey — terraced house | £95k–£160k | £120k–£200k |
| 1 storey — semi-detached | £100k–£170k | £125k–£210k |
| 1 storey — detached house | £115k–£190k | £140k–£230k |
| 2 storeys — detached house | £145k–£265k | £180k–£320k |
Cost per m² for upward extensions
The high fixed cost of structural works means upward extensions make better financial sense on larger houses where the new floor area is maximised. A large detached house adding two storeys achieves a more acceptable cost-per-m² than a small terraced house adding one.
Professional fees — upward extensions
Structural Challenges: Why It Costs So Much
The idea of an upward extension sounds straightforward: take off the roof, add another floor, put a new roof back on. The reality is substantially more complex for almost every London house.
Foundations almost never designed for additional load
Victorian and Edwardian terraces — which make up the majority of London's housing stock — were built with shallow strip foundations in London clay. These were sized for the original building load. Adding one or two storeys increases the structural load on the foundations by 30–70%. A structural engineer must assess whether the existing foundations can take this load, or whether they need to be underpinned or supplemented. Foundation work can represent £20,000–£50,000 of the project cost on its own.
Temporary propping works
Once the roof is removed, the top of the existing walls must be temporarily supported (propped) while the new structure is built. Propping a London terrace is an engineering operation in itself. The props must transfer load safely down through the building. This temporary works design adds cost and means the building is essentially opened to the weather and cannot be occupied safely during the structural phase.
Wall strength and lateral stability
Adding height to a building changes its lateral stability (its resistance to wind load). A two-storey Victorian terrace was designed to resist a certain wind envelope. Three storeys creates a taller, heavier structure. The structural engineer must check wall thickness, wall ties, lateral restraints, and may specify additional steelwork to stiffen the structure laterally.
New staircase
The new storey requires a compliant staircase up from the existing top floor. Where space allows, this fits within the existing layout. In a typical London terrace, a new staircase in the right position will take floor space from one of the existing upper bedrooms. Staircase design and position is one of the critical design decisions in any upward extension.
Services rerouting
Heating, plumbing, and electrical services must be extended to the new storey. In most London houses, the boiler and hot water cylinder are already at the upper limits of their capacity; a new floor often requires a new or larger boiler, new pipework runs, and additional electrical consumer unit capacity.
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Practical Limitations in London
Beyond the structural and planning considerations, upward extensions in London face a set of practical constraints that rule them out for many projects:
Crane access
Removing a roof in a London street and bringing in structural steelwork often requires a crane. Narrow terraced streets may not have room for a crane, or may require road closures and traffic management licences. This adds cost and programme time, and may be refused by the local council.
Living in the house during works
Occupying a house while the roof is off and temporary propping is in place is not safe or practical. Most upward extension projects require the occupants to vacate for the structural phase — typically 10–16 weeks. In London, temporary rental costs can easily reach £15,000–£30,000 for a family-sized house, and this must be budgeted as part of the project cost.
Neighbour relations
Even where prior approval is obtained, neighbours of a terraced or semi-detached house will experience significant disruption. Party wall notices must be served. Craning operations affect the whole street. Contractors' vehicles and skips occupy road space for months. In closely built London streets, neighbour management is a serious project consideration.
Sequential programme — no phasing
Unlike a rear extension where the house remains largely habitable while the extension is being built, an upward extension involves the whole building simultaneously. There is no easy way to phase the works so that the house remains usable during construction.
When an Upward Extension Makes Sense
Given the cost and disruption, an upward extension is not the obvious first choice for most London homeowners. But there are specific circumstances where it is the right solution:
The garden is too small for a horizontal extension
In London, many smaller terraced houses already have very limited rear gardens. A rear extension that meets permitted development limits may create useful space, but also eliminates most of the outdoor space. An upward extension preserves the garden entirely. For families who value outdoor space, this is a compelling reason to go up rather than out.
High-value areas where value uplift justifies the cost
Adding a bedroom storey to a house in a prime London location — where additional bedrooms add very significant value — can produce an excellent return. A large detached house in Kensington or Notting Hill adding two storeys under Class AA may add £500k–£1.5m in value against a build cost of £250k–£320k. The arithmetic only works in high-value areas.
Purpose-built flat blocks seeking to add units
For the freeholder of a purpose-built block of flats, Class AB offers the prospect of creating new dwellings without a full planning application. The new units can be sold to fund the works and potentially yield a significant profit, while the existing leaseholders benefit from an improved building. This is a developer or freeholder-led use case, not a single homeowner project.
The Permitted Development Extension Bill
Lord Lucas introduced the Permitted Development (Extension) Bill in the House of Lords in 2024, proposing to extend and simplify PD rights — including Class AA and AB. The Bill proposes expanding the height limits, simplifying the prior approval criteria, and extending Class AA to cover semi-detached and terraced houses on the same terms as detached houses (i.e., allowing two additional storeys rather than one).
As of February 2026, the Bill has not been enacted. The existing Class AA and AB rules remain in force. Homeowners planning projects in 2026 should work within the current rules and not anticipate legislative changes that have not yet been made law.
Common Mistakes
Assuming Class AA applies without checking
The most common error. Conservation areas alone affect a large proportion of inner-London houses. Check your property's listing status, conservation area boundary, and Article 4 directions before commissioning a structural engineer. The checks take minutes; discovering the right does not apply after spending £5,000 on engineering reports is costly.
Underestimating structural costs
Online budget calculators for upward extensions often quote cost-per-m² figures similar to a conventional extension. This misrepresents the economics. The fixed structural cost of an upward extension — temporary works, foundation assessment or underpinning, wall strengthening — is a large sum that applies regardless of the floor area. Budget the structural costs separately before calculating cost-per-m².
Not budgeting for temporary accommodation
Vacating a London house for 16–24 weeks is expensive. Families often underestimate the cost of renting equivalent accommodation in London while works proceed. Temporary accommodation should be included in the feasibility budget from the outset, not discovered as an additional cost after the project has been committed.
Treating prior approval as equivalent to full planning permission
Prior approval under Class AA is a more limited assessment than full planning permission, but it is still a formal decision by the council. A prior approval refusal is a planning refusal and goes on the public register. It can also affect future planning applications for the property. Seek proper pre-application advice from an architect before submitting.
Ignoring the staircase problem
A new storey must be accessed by a compliant staircase. In many London terraced houses, the only place for a new staircase will eat significantly into an existing bedroom or landing. This is sometimes a dealbreaker: the new storey creates one bedroom but destroys one existing bedroom, leaving the net gain as zero. Plan the staircase position before committing to the project.
Frequently Asked Questions
What is Class AA permitted development for upward extensions?
Class AA is a permitted development right introduced in August 2020 that allows homeowners to add one or two storeys on top of an existing house without full planning permission. Detached houses can add up to 2 storeys (not exceeding original roof height plus 7m or 18m total). Semi-detached and terraced houses can add up to 1 storey (not exceeding original roof height plus 3.5m). Prior approval from the council is always required.
How much does an upward extension cost in London in 2026?
All figures inc VAT: adding one storey above a terraced house costs £120k–£200k all-in; one storey above a semi-detached costs £125k–£210k; one storey above a detached costs £140k–£230k; two storeys above a detached costs £180k–£320k. Cost per m² is £4,500–£7,000 due to high fixed structural costs. Temporary accommodation adds a further £8,000–£25,000 for most projects.
Do I need planning permission to add a storey to my house in London?
Not necessarily. Class AA permitted development allows you to add storeys without full planning permission, but you always need prior approval — a mandatory check by the council before works begin. However, Class AA does not apply in conservation areas, to listed buildings, or where an Article 4 direction has removed the right. A large proportion of inner-London houses are in conservation areas and therefore need full planning permission.
What is prior approval for an upward extension?
Prior approval is a mandatory council check required before any Class AA or AB upward extension can proceed. The council assesses external appearance, impact on neighbours, structural soundness, and highway impacts. Neighbours are consulted for 21 days. The council has 8 weeks to decide. The application fee is £234. Prior approval does not replace building regulations approval, which is also required.
Can I add a storey to a flat in London?
Class AB permits upward extensions on purpose-built blocks of flats built between 1 July 1948 and 5 March 2018, allowing up to 2 additional storeys subject to prior approval. However, this is a complex exercise for leasehold flat blocks: the agreement of freeholders and all leaseholders is typically required in addition to planning approval. Conservation area and listed building exclusions also apply.
Can I live in my house during an upward extension?
Not during the structural phase. Removing the roof and installing temporary propping requires the building to be vacated. The structural phase typically takes 10–16 weeks. Most upward extension projects in London require occupants to find alternative accommodation for 4–6 months total. This cost — typically £8,000–£25,000 for London rents — must be included in the project budget.
Is an upward extension worth it in London?
It depends on the circumstances. The case is strongest where the garden is too small for a horizontal extension, and in high-value areas where the additional bedrooms create significant property value uplift. The case is weaker for smaller terraced houses where the fixed structural cost is high relative to the value gain. A detailed feasibility assessment — comparing the cost of going up against a loft conversion or rear extension — is essential before committing.
What properties are excluded from Class AA?
Class AA does not apply to: listed buildings (any grade), properties in conservation areas, properties subject to an Article 4 direction removing these rights, properties within 1km of a licensed aerodrome (without CAA consultation), properties on a Site of Special Scientific Interest, and houses built after 28 October 2018. In inner London, conservation areas alone exclude a very large proportion of the housing stock.
Summary
Class AA and AB permitted development rights offer a route to add storeys to London houses and flat blocks without full planning permission — but prior approval is always required, and the exclusions (conservation areas, listed buildings, Article 4 directions) rule out a large proportion of inner-London properties.
The structural reality is considerably more demanding than the planning framework suggests. Existing foundations, wall strength, temporary propping, crane access, and the need to vacate during works all contribute to a cost base that is much higher per square metre than a conventional extension. Budget £120k–£200k for one storey above a terraced house; £180k–£320k for two storeys above a detached.
Start by checking whether Class AA applies to your property — particularly conservation area status and Article 4 directions — before commissioning any professional fees. If the planning route is available, commission a structural engineer's feasibility assessment before committing to a full design. The structural costs are the dominant variable in every upward extension project.
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