Adding another storey can unlock significant additional floor space without increasing the footprint of a property. For homeowners, it can mean another bedroom, bathroom or living area. For developers and owners of larger buildings, extending upwards can sometimes create additional homes from underused roof space or airspace.
But an upward extension is not simply a larger version of a loft conversion.
Before an additional floor can be designed properly, three questions need answering: does the planning route allow it, can the existing building safely carry it, and how will the enlarged building meet current Building Regulations?
In England, certain houses and other qualifying buildings can benefit from permitted development rights for upward extensions. These rights are subject to detailed limitations and, importantly, prior approval from the local planning authority. Other buildings will require a full planning application.
The structural assessment is equally important. An existing building may never have been designed to carry another complete floor, so its walls, beams, columns and foundations need to be understood before the additional load is committed to the design.
Can You Add Another Storey to an Existing Building?
Yes, in principle. Existing buildings can be extended upwards, but the correct route depends on the type of building, its age and use, its location, the proposed height and what the new floor will be used for.
There are two important permitted-development routes to distinguish.
For an existing house, Class AA of Part 1 of the General Permitted Development Order can allow the house itself to be enlarged upwards, subject to all applicable limitations, conditions and prior approval.
Separately, Part 20 contains permitted development rights that can allow new self-contained homes to be created above certain existing buildings, including qualifying purpose-built detached blocks of flats and some residential, commercial and mixed-use buildings.
These are not interchangeable routes.
A homeowner adding another floor to their existing house is dealing with a different set of provisions from a developer proposing new flats above an existing mixed-use building.
That distinction should be established at the beginning of the project.
Do You Need Planning Permission to Add Another Storey to a House?
Not always in the form of a full householder planning application.
Under Class AA of Part 1, certain houses in England can be extended upwards under permitted development rights, but the development is still subject to an application for the local planning authority’s prior approval.
The current Planning Portal guidance for additional storeys states that, for this householder route, the existing house must satisfy several eligibility requirements. Among other restrictions, it must have been constructed between 1 July 1948 and 28 October 2018, must not already have had additional storeys added under the relevant provision, and cannot be on Article 2(3) land such as a Conservation Area. The right also does not apply to a building containing flats or to a flat within such a building.
For qualifying houses, the national rules can allow:
|
Existing house |
Potential upward extension under Class AA |
|
Single-storey house |
Up to one additional storey |
|
House with two or more storeys |
Up to two additional storeys |
That does not mean every one- or two-storey addition automatically qualifies.
There are further restrictions covering total height, the relationship with adjoining buildings, floor-to-ceiling height, the principal part of the house, materials, roof form, engineering works and windows.
Planning Portal currently states that the resulting house cannot exceed 18 metres in total height under this householder route. An additional storey cannot exceed the relevant height limits, and the internal floor-to-ceiling height of a new storey cannot exceed the lower of 3 metres or the floor-to-ceiling height of an existing storey in the principal part of the house.
For a semi-detached or terraced house, the relationship with the height of adjoining buildings also matters.
The full eligibility test should therefore be carried out against the actual building rather than relying only on the number of storeys proposed.
AC Design Solution’s Permitted Development Checker can provide an initial indication of the likely planning route for residential projects.
What Is Prior Approval for an Additional Storey?
Permitted development does not mean that an eligible upward extension can simply be built without involving the council.
For Class AA upward extensions to houses, prior approval must be obtained before development begins.
The local planning authority can consider specified matters including the impact on adjoining premises through overlooking, privacy and loss of light; the external appearance of the house; air traffic and defence asset impacts; and effects on identified protected views. The legislation also requires construction-management information addressing matters such as working hours and the mitigation of noise, dust, vibration and traffic impacts.
This makes prior approval substantially different from assuming that a project is automatically acceptable simply because it appears to fit within a height limit.
The design still needs to respond to the existing building and its surroundings.
When Will Full Planning Permission Be Required?
If the proposal does not satisfy the permitted-development criteria, the appropriate route may be a full planning application.
For a house, this could happen because of its age, location, previous development, proposed height or another Class AA limitation.
Conservation Areas are particularly important. Planning Portal identifies Conservation Areas as Article 2(3) land, where the householder Class AA upward-extension right does not apply. Other designated land identified by the rules is also excluded.
A full planning application does not simply ask whether the development fits the permitted-development dimensions. The local planning authority assesses the proposal against the relevant development plan and other material planning considerations.
For an additional storey, the design may need to respond carefully to the existing roofline, architectural character, neighbouring buildings, street composition, overlooking and the overall scale of the enlarged building.
That is why the planning strategy and architectural design should be developed together.
What About Adding Storeys Above Flats or Commercial Buildings?
The rules are different from adding living space to an individual house.
Part 20 of the General Permitted Development Order contains several rights for creating new dwellinghouses above qualifying existing buildings.
Government planning guidance confirms that these rights can apply to certain purpose-built detached blocks of flats and certain commercial, mixed-use and residential buildings. Depending on the particular class and existing building, up to two additional storeys may potentially be available, subject to detailed limitations, conditions and prior approval.
For example, Part 20 Class A relates to new dwellings above qualifying purpose-built detached blocks of flats. Other Part 20 classes deal with particular detached or terraced commercial, mixed-use and residential buildings.
The eligibility tests are detailed and differ between the classes.
For larger development opportunities, it is therefore important to establish the existing building’s lawful use, construction date, height, number of storeys, form and planning history before treating its roof as developable airspace.
A building having physical room for another floor does not by itself mean the planning system allows one.
Structural Feasibility Should Be Checked Early
Planning is only half of the question.
Adding a new floor introduces permanent additional loads to a building that may have been designed decades earlier for a very different structural arrangement.
Those loads need a continuous route from the new construction down to the ground.
This is the load path.
A new floor may be supported by existing masonry walls, new beams, an existing reinforced-concrete frame or another structural system. Whatever the solution, each supporting element needs to be capable of transferring the additional forces safely through the building.
Planning Portal’s guidance on existing walls and foundations for upward extensions specifically highlights the need to consider whether existing walls, beams and foundations can support the additional loads.
This is why an early assessment by a house structural engineer can be valuable before a design becomes fixed.
Can the Existing Foundations Support Another Floor?
Sometimes they can. Sometimes strengthening or a different structural strategy may be required.
There is no reliable answer based simply on the age or appearance of the building.
The engineer needs to understand how the existing structure works and how the proposed loads will reach the foundations.
An older house may have load-bearing masonry walls and foundations that were never designed with an additional storey in mind. A larger apartment or commercial building may instead have a reinforced-concrete or steel frame with a different distribution of structural capacity.
Previous alterations also matter.
For example, an original load-bearing wall may have been removed at ground-floor level and replaced with a beam. Adding another floor above can increase the load carried by that beam and its supports.
Planning Portal specifically identifies this situation in its upward-extension guidance and recommends checking whether existing beams and foundations are adequate for the additional loads.
The important point is that the new storey cannot be designed in isolation from everything underneath it.
Why Lightweight Construction Can Help – But Does Not Remove the Need for Engineering
Upward extensions are often associated with relatively lightweight construction because reducing the additional dead load can be beneficial when working with an existing structure.
But specifying a lighter superstructure does not automatically prove that the building can carry the extension.
The structural engineer still needs to assess the new floors, roof, walls, beams or frame, connections, lateral stability and the existing structure below.
There may also be local strengthening requirements where new loads are concentrated.
The right structural solution therefore comes from understanding the existing building first, rather than choosing a construction system solely because it is described as lightweight.
Real Project Experience: Ealing Broadway W5
AC Design Solution’s Ealing Broadway W5 project demonstrates the type of coordination involved when extending an existing building upwards.
The project at 134 Broadway, Ealing involved an additional fifth floor to an existing mixed-use building containing 19 flats. Approximately 90 square metres of additional floor area was created.
Unlike extending outward onto clear ground, an upward extension has to work with the building already beneath it. The existing structural arrangement, new loads, architectural proportions and planning requirements all influence what can realistically be added.
The Ealing Broadway project involved both planning and structural challenges, making coordination between the architectural and engineering work central to developing the additional floor.
It is a useful example of why airspace should not be assessed purely by looking at an apparently unused roof. The opportunity above a building is directly connected to the structure and constraints below it.
Building Regulations for an Additional Storey
Whether an upward extension proceeds through permitted development, prior approval or full planning permission, Building Regulations remain a separate requirement.
GOV.UK makes clear that planning permission and Building Regulations approval are different systems.
Planning Portal states that Building Regulations approval is required when additional storeys are added to an existing building to create liveable space. Relevant considerations can include structural strength and stability, fire safety, stairs, sound insulation, ventilation, drainage and energy performance.
For a coordinated technical package, AC Design Solution’s Building Regulation Drawings service develops the construction information required for Building Control review alongside the relevant structural design.
Planning approval should therefore never be treated as confirmation that the structure or construction details comply with Building Regulations.
Fire Safety Can Affect More Than the New Floor
An additional storey changes the height and internal arrangement of a building.
That can change the fire-safety strategy beyond the footprint of the new construction itself.
For houses, Planning Portal explains that escape provisions need to be considered throughout the full escape route when another storey is added. Depending on the building, this can mean upgrading protection within existing parts of the property, including the stair enclosure, doors, alarms and elements of the existing structure.
A suitable staircase is also required to serve the new level, and creating the stair opening affects the floor structure between the existing and new storeys.
Current statutory fire-safety guidance for England is contained in Approved Document B.
For larger residential or mixed-use buildings, fire strategy can become considerably more involved, particularly as building height increases.
Could Adding a Storey Make the Building a Higher-Risk Building?
This is an important early check for larger residential projects.
Under the current building-safety regime in England, a building can fall within the higher-risk building regime during building work where it meets the relevant use criteria and is at least 18 metres high or has at least seven storeys. GOV.UK specifically notes that building work to an existing building that makes it a higher-risk building can bring the project within the Building Safety Regulator’s building-control regime.
That means adding a floor to a building close to one of these thresholds should not be approached as a routine roof extension.
Where the higher-risk building criteria are met, the Building Safety Regulator is the building control authority for relevant work and approval is required before applicable building work starts.
This issue should be identified at the feasibility stage, because it can materially affect the design, information requirements and approval process.
Does the Party Wall Act Apply When Adding Another Storey?
It can.
The Party Wall etc. Act 1996 is separate from planning permission and Building Regulations.
For terraced and semi-detached properties in particular, an upward extension may involve work to an existing party wall or party structure.
The government’s Party Wall etc. Act 1996 explanatory booklet specifically identifies raising the height of a party wall, for example when adding another storey, as one of the types of work covered by section 2 of the Act.
The exact Party Wall requirements depend on the proposed works.
Where applicable, notices need to be dealt with at the correct stage before the relevant work begins. AC Design Solution’s Party Wall surveying service can be coordinated with the architectural and structural design where the Act applies.
Prior Approval Does Not Replace Structural Design
This distinction is particularly important.
A local planning authority granting prior approval does not mean that the council has confirmed the existing foundations can carry another floor.
Equally, a structural engineer confirming that an additional storey is technically feasible does not mean that it will receive planning permission or prior approval.
These are different assessments.
For a successful upward-extension project, the planning and structural questions need to be investigated alongside one another.
There is little benefit in developing a planning proposal for an additional floor that would require an impractical structural intervention. Equally, there is little benefit in completing detailed structural calculations for a design that does not have a viable planning route.
What Should Be Checked Before Designing an Additional Storey?
The most efficient starting point is a combined planning and structural feasibility review.
The review should establish the existing building type and use, construction date, planning history, number of storeys and current height. It should identify whether the property is within a Conservation Area or another designated area and whether an Article 4 Direction or existing planning condition affects the available permitted development rights.
At the same time, the existing structural form should be understood sufficiently to identify the likely load paths, supporting walls or frame, significant previous alterations and potential foundation implications.
For larger buildings, the proposed final height and number of storeys should also be checked against the higher-risk building criteria at the outset.
Once those fundamentals are understood, the architectural design can be developed around a planning route and structural strategy that are both realistic.
Additional Storey vs Loft Conversion: What Is the Difference?
A loft conversion normally adapts or enlarges space within the roof zone.
An additional-storey project creates a new storey immediately above the existing topmost storey and can involve replacing or substantially reconstructing the existing roof.
The distinction matters in planning terms.
Class B of Part 1 deals with certain additions and alterations to the roof of a dwellinghouse, while Class AA deals with enlargement by constructing additional storeys. The eligibility criteria and limitations are different.
A proposal should therefore be assessed according to what is actually being constructed rather than simply describing every project above the top floor as a “loft conversion”.
Is Airspace Above a Building Automatically Developable?
No.
A roof may look underused, particularly on a flat-roofed apartment or commercial building, but physical space does not establish planning or structural feasibility.
An airspace development opportunity still depends on the planning route, building use, existing height, architectural context, neighbouring amenity, structural capacity, access, fire safety, services and Building Regulations.
For qualifying buildings, national permitted development rights may create an opportunity. For others, a full planning application may still support an upward extension where the design responds successfully to the building and local planning context.
The starting point should be evidence, not an assumption based on the roof area alone.
Frequently Asked Questions
Can I add another storey to my house without full planning permission?
Potentially. In England, Class AA permitted development rights can allow certain houses to be extended upwards, subject to detailed limitations and mandatory prior approval. The house and proposed development must satisfy all applicable criteria.
How many storeys can I add under permitted development?
For qualifying houses under Part 1 Class AA, a single-storey house can potentially gain one additional storey, while a house with two or more storeys can potentially gain up to two. Height and other restrictions still apply. Different rules apply to projects creating new dwellings under Part 20.
Can I add a storey to a house in a Conservation Area under Class AA?
The householder Class AA right does not apply on Article 2(3) land, which includes Conservation Areas. A different planning route would therefore need to be considered.
Can another floor be added above a block of flats?
Potentially. Part 20 includes permitted development rights for new homes above certain qualifying buildings, including purpose-built detached blocks of flats, subject to detailed eligibility requirements, limitations and prior approval. Where the permitted-development route does not apply, a full planning application may still be considered.
Do I need a structural engineer before applying for planning permission?
A structural design is not the same as planning approval, but an early structural feasibility assessment can be particularly valuable for an upward extension. It helps establish whether the existing walls, frame, beams and foundations are likely to accommodate the additional loads and identifies major structural constraints before the architectural design is fixed.
Will the existing foundations need strengthening?
Not necessarily. It depends on the existing foundations, ground conditions, structural arrangement and additional loads. Some buildings may have sufficient capacity; others may require strengthening or a different structural strategy. This requires project-specific engineering assessment.
Do I need Building Regulations approval if the additional storey is permitted development?
Yes. Permitted development relates to the planning route. Building Regulations are separate, and an additional liveable storey requires Building Regulations compliance.
Can adding another storey trigger the higher-risk building regime?
Potentially. If the work makes an existing building meet the relevant higher-risk building criteria, including the applicable height/storey and use thresholds, the Building Safety Regulator regime may apply. This should be checked early on larger residential projects.
Do I need to tell my neighbour before adding another storey?
It depends on the work. If the project involves notifiable work under the Party Wall etc. Act 1996, such as raising an existing party wall, the relevant notice procedure applies. Planning neighbour consultation and the Party Wall process are separate matters.
Planning an Upward Extension
Adding another storey can make better use of an existing building without increasing its footprint, but the opportunity has to be tested from more than one direction.
The planning route establishes whether the additional floor is acceptable through permitted development and prior approval or whether full planning permission is required. Structural engineering establishes whether the existing building can safely support the new construction. Building Regulations then address how the enlarged building will perform
