Adding a storey to a building: a structural guide

Can you add a storey to an existing building?

Often, yes. Many London buildings were designed with spare capacity, or can be strengthened at reasonable effort, and a lightweight new storey adds less load than people expect. The engineer's job is to find out how much load the existing structure can safely take, then design a new storey that stays within it.

We work on homes, flats, conversions and mixed-use buildings up to 15 m high and five storeys, measured with the new storey in place. That covers most upward extensions on London terraces, semi-detached houses and small purpose-built blocks of flats.

An extra storey can make a lot of sense in a city where land is scarce. It adds a home or a larger home without a bigger footprint, and it can pay for repairs to the roof and common parts at the same time. But the structural questions come first, before the money is spent on drawings.

Planning routes

Adding a storey needs planning consent of some kind. In some cases, upward extensions to houses and certain blocks of flats can go through a prior approval process under permitted development, which limits what the council can consider. In others, a full planning application is needed. The council will confirm which route applies.

Either way, the council will look at daylight to neighbours, overlooking, the appearance of the street and, for flats, the impact on existing residents. They may ask for a structural statement showing the building can be extended safely, and our structural planning statement is written for that purpose.

Structural capacity: the first question

Before any design starts, the engineer needs to understand the existing building properly. That means a survey of the walls, floors and roof, looking for past alterations, and finding out what the foundations actually are. Original drawings help if they exist, but they rarely show the building as built.

Trial pits at a few points around the building reveal the depth and width of the footings and the ground beneath, which in much of London is clay. The engineer then carries out a load take-down for the existing building and the proposed one, and compares the extra load with the capacity of the walls, piers, beams and foundations.

The outcome is usually one of three things: the building can take the new storey as it is, it can take it with local strengthening, or it cannot take the proposed form at all and a lighter or smaller scheme is needed. Our structural capacity appraisal sets this out clearly before you commit to design fees.

Choosing a lightweight structure

Weight is the single biggest factor in whether an upward extension works. Heavy masonry and concrete floors are rarely used for a new top storey. The common options are light gauge steel framing, timber frame, cross-laminated timber panels and hot-rolled steel frames with lightweight floors.

Each has trade-offs. Lighter systems put less load on the building but need more care with fire resistance, sound insulation between flats and stiffness in wind. Prefabricated systems reduce time on site, which matters when residents are living below.

The new structure also needs a way to spread its load onto the existing walls. That is often a steel grillage or a ring beam at roof level, which ties the top of the existing building together and gives the new frame a level base.

Robustness and disproportionate collapse

Building Regulations Part A requires a building to be robust, so that damage to one part does not lead to a collapse out of proportion to the cause. The measures needed depend on the building's consequence class, which is set by its use, number of storeys and floor area.

Adding a storey can move a building into a higher consequence class. For a block of flats, going from four storeys to five is a common example, and it can bring requirements for horizontal and vertical ties or a check of key elements through the whole building, not just the new floor. The engineer should identify this at feasibility stage, and a disproportionate collapse assessment covers it where needed.

The existing building's resistance to wind also needs checking. A taller building catches more wind, and older blocks sometimes rely on internal walls that have since been removed or cut through. Where stability is marginal, the new storey may need its own bracing, or the existing building may need new shear walls or frames.

Fire, stairs and staying within 15 m

A new storey changes fire safety as much as structure. The stair usually has to be extended upwards, and in a block of flats the common stair must continue to provide a protected escape route for everyone. Fire separation between the new and existing floors has to meet the regulations, and the structure must keep its strength for the required period.

Height matters too. The rules become more demanding as buildings get taller, so the height after the extension needs to be measured carefully at feasibility stage. We only take on schemes that remain within 15 m and five storeys once the new storey is added.

Working above occupied homes

Many upward extensions are built while people live below. The roof is opened up, the new structure is installed and the building must stay weathertight throughout. A temporary roof on scaffolding is common, and the sequence of work is planned so existing homes are disturbed as little as possible.

Neighbouring buildings are often affected too, through raised party walls or new loads near the boundary, so party wall notices are usually needed. A construction management plan, often required by the council, sets out deliveries, working hours and how residents will be kept safe.

Ownership is the other practical point. In a block of flats, the roof and airspace usually belong to the freeholder, and leaseholders below may have rights that affect the work. Agreeing these early, with legal advice, avoids a structurally sound scheme being held up by a dispute over access or consent.