Structural design for extra storeys and upward extensions
Feasibility and structural design for adding one or more storeys on top of a house, block of flats or mixed-use building, including checks on the existing structure and foundations.
About this service
Building upwards is one of the few ways to add homes in London without finding new land. Adding a storey to a house, a mansion block or a parade of shops uses roof space you already own, but every new floor adds weight to walls and foundations designed for the building as it stands. Our upward extension service first establishes whether the existing structure can take that weight, then designs the new storeys and any strengthening needed.
We work for freeholders of small blocks, developers adding flats above shops, and homeowners adding a full storey to a house. Our scope covers buildings that, once extended, are still no more than 15 metres high and five storeys. In practice that usually means taking a two or three-storey building up by a floor, or a four-storey block to five where the height allows.
London buildings vary enormously. A 1930s block may have a concrete frame with spare capacity, while a Victorian parade may sit on shallow brick footings in London Clay with no drawings at all. That is why feasibility comes first: it tells you whether the scheme works before you spend heavily on planning drawings. Every set of calculations then goes through our three-step quality control process, with final sign-off by our Head of Engineering.
When you need it
- You want to build extra flats on the roof of a block or above a parade of shops
- You are adding a storey to a house or mansion block
- You need to know whether the existing structure and foundations can take the extra load
What's included
- Investigation of the existing structure and foundations
- Load assessment for the additional storeys
- Lightweight frame options in steel or timber
- Design of strengthening, new frame and connections
- Calculations and drawings for planning, Building Control and construction
How it works
- Gather records and inspectWe collect existing drawings, Building Control records and earlier surveys, then inspect the building to establish wall construction, floor spans, roof structure and any visible signs of past movement.
- Investigate the foundationsWe specify trial pits and, where needed, boreholes or opening-up for a contractor to carry out. These reveal the real size and depth of the foundations and the soil beneath, which drives the whole feasibility.
- Assess the extra loadWe calculate the existing loads and the added weight of the new storeys, then check walls, columns and foundations. We compare lightweight steel and timber options to keep the added load as low as possible.
- Report on feasibilityYou receive a feasibility report stating whether the extension is achievable, the frame options, the likely strengthening and the main risks, so you can decide before committing to a planning application.
- Detailed designWhen you proceed, we design the new frame, connections, transfer elements and strengthening, and produce calculations and drawings for planning, Building Control and construction.
What you receive
Choosing a frame and dealing with the existing building
Weight is the main design driver. Light gauge steel framing, timber frame and cross-laminated timber are all far lighter than masonry or concrete, and are often the only way to add a storey without underpinning. Prefabricated panels also cut the time spent working on the roof, which matters when residents live below. The right choice depends on spans, the fire strategy, acoustic separation and how the new loads can be brought down onto existing walls.
Adding a storey can change how the building is treated under Building Regulations Part A for disproportionate collapse. A block of flats going from four to five storeys, for example, moves into a higher consequence class, which can require better ties through the existing structure, not just the new floor. We check this at feasibility stage because it can affect the cost of the whole scheme, and our disproportionate collapse assessment covers it in detail where needed.
Practical issues follow quickly: party wall notices, scaffold and crane access, keeping the existing roof weathertight, and protecting residents during the works. Planning routes vary, and permitted development rights for upward extensions only apply in certain cases, so your council will confirm what applies. If you just need to know whether the structure can cope, a structural capacity appraisal is a lighter first step.
Not included
- Ground investigation and opening-up works by the contractor
- Party wall surveyor fees
Frequently asked questions
Can I add another storey to my house?
Often you can, but it depends on the existing walls and foundations. Many London houses have shallow brick footings sized for the original load, and an extra storey can push them beyond what the soil can safely carry. We investigate the foundations, calculate the loads and tell you whether the storey is feasible and what strengthening it would need.
Do I need to underpin to add a storey?
Not always. If the new storey is light and the existing foundations have some spare capacity, underpinning can often be avoided. Where foundations are shallow, sit on shrinkable London Clay near trees or already show movement, some underpinning or new foundations may be needed. Trial pits give the answer, and our underpinning design service follows on if it is required.
Can you add storeys to any block of flats?
We work on buildings that remain within 15 metres in height and five storeys once extended. Within that range, feasibility depends on the existing structure, the foundations, the escape stairs and the overall stability of the building. Some blocks have plenty of reserve capacity and others very little. A feasibility report is the reliable way to find out before investing in a planning application.
How much does a feasibility study for a roof-top extension cost?
Our additional storeys and upward extensions service starts from £1,500 + VAT. The fee depends on the size of the building, the records available and how much investigation is needed. Ground investigation and opening-up are carried out by a contractor and are not included. You will have a fixed fee confirmed in writing before any work starts.
Will residents have to move out during an upward extension?
Often they do not, particularly where prefabricated lightweight frames limit the time spent working on the roof. The roof must stay weathertight throughout, and any strengthening inside existing flats needs careful planning and access agreements. The contractor's method statement and temporary works should deal with resident safety, noise and access before work begins.