Can an existing building take additional floors and how do I find out? 

Yes, in many cases an existing building can be extended upwards, but whether your building can or not depends on a specific set of structural factors: its foundations, its original construction usage, how it was designed to carry load, and how it will behave once extra height changes its stability. The only reliable way to know whether these structural factors have been met is a structural feasibility appraisal, carried out before the scheme is fixed and ideally before a planning application goes in. 

Of course, structural feasibility isn’t the only consideration. Getting a vertical extension from idea to consented, buildable scheme is a wider project and usually runs in parallel with input from planning consultants, architects, and M&E engineers.  

This article covers what determines whether a building can take additional floors from a structural standpoint, how the appraisal process works, and an example project where getting this right was the difference between a viable scheme and a costly redesign.  

What Actually Determines the Answer 

Whether a building has spare capacity or not often isn’t discernible from a site visit alone. Below outlines what actually determines the answer: 

Foundations – Adding storeys adds load, and that load has to go somewhere. To understand whether the existing foundations can take more load, you would need to obtain detailed ground information and undertake a geotechnical assessment. The existing foundations tend to be the limiting factor rather than the frame above ground, and further foundation strengthening is often expensive, time consuming or just not feasible. 

Construction type and era – A Victorian masonry building, a 1960s concrete frame, and a steel-framed building from the interwar period all behave differently under additional load and were often designed to entirely different allowances for material strength and safety margins. Knowing the era tells you roughly what questions to ask, but not the answer. 

Original design approach – The codes of practice used to design older buildings, which predate today’s codes may not account for ‘modern’ considerations. Checking an existing structure for new loading means working in the design language the original engineers used, not the one in use today. You’ll find more detail on this below. 

Lateral stability – Adding height changes how a building attracts and resists wind load, not just how it carries vertical weight. A frame with adequate vertical capacity can still fail a stability check once made taller. Extra storeys can also shift the building’s disproportionate collapse category, meaning the level of robustness needed so that damage to one element doesn’t trigger a wider chain reaction. That’s a separate check from day-to-day strength and stability, and it’s easy to miss if it isn’t looked at explicitly. 

Party walls and neighbouring structures – This is particularly relevant for terraced or tightly-bounded urban sites, where load paths and stability may be shared with adjoining buildings. 

Accessible rooftop space – If your new roof is being designed as usable amenity space, it will have its own loading allowance on top of the extension itself. Barriers, planters, seating areas and footfall all need to be accounted for, and this is a common one to underestimate at an early stage. 

Change of use – If the use of the building is going to change as part of the project, it could help or hinder the structural feasibility as the potential maximum load from the existing floors may change. This can work in a scheme’s favour: a change of use to something with a lower imposed load allowance than the original design, residential in place of office storage or retail, for example, can free up capacity elsewhere in the structure and help offset the extra load from the new storeys. Vertical extensions and change of use are often proposed together for exactly this reason. 

Why This Matters Most at Planning Submission Stage 

You may be tempted to treat structural feasibility as something to confirm once a scheme is largely fixed. However, for vertical extensions, that’s the wrong order. If the structure can’t support what’s being proposed, or can only support a reduced version of it, finding that out after a planning application has gone in means redesign, delay, or a scheme that doesn’t match what was consented. 

Getting a structural appraisal early doesn’t slow your planning submission down. It usually protects your timeline, because the design team is working to confirmed structural allowances rather than assumed ones. It might even be the case that more floors can be added than originally planned! 

How the Appraisal Process Actually Works 

Not to be confused with a structural assessment or building appraisal, a structural appraisal or structural feasibility appraisal follows a consistent sequence: 

1. Desk study – reviewing any existing drawings, calculations, or historical records for the building should these exist. 

2. Site survey – confirming what’s actually there, including element sizes and general condition, particularly where records are incomplete or missing. This also often includes breakouts and material testing. 

3. Structural assessment – checking the existing structure against the proposed additional loading, using an approach consistent with how the building was originally designed, but that also encompasses modern considerations like robustness and wind loading. 

4. Ground investigation –  where foundation capacity is in question. 

5. Feasibility report – a clear position: yes, no, or conditional, with the reasoning and any strengthening implications set out. 

Where It Gets Complicated: A Vertical Extension in London 

To illustrate the process, Link Engineering and MHA Structural Design carried out the structural feasibility appraisal on a mid-century hotel building in London (a steel-framed amalgamation of several buildings developed between roughly 1950 and 1965), which needed to support a two-storey vertical extension plus full-height extensions at both ends.  

The complication was that the code the original frame was designed to, BS449, a permissible stress approach used from 1932 to 1985, was fundamentally different from the limit state design (Eurocode 3) used today. Standard modern design tools couldn’t check the existing frame on their own.  

The solution was a bespoke spreadsheet based on the SCI P440 methodology, bridging Eurocode-based analysis with BS449’s original assumptions, backed by on-site verification of section sizes and connection details. The result was an efficient, defensible check of the existing structure that would have been far more difficult to achieve working through BS449 in isolation.  

Common Misconceptions 

“If it’s a steel frame, it can automatically take more load.” Whilst a steel frame often has a well defined load path, that alone says nothing about the original design margin or the code it was designed to. 

“If drawings exist, we don’t need a survey.” Original drawings, where they exist at all, rarely tell the full story of what was actually built, particularly in buildings altered or extended over time. 

“Structural feasibility work will hold up planning.” Done early and run in parallel with design development, it typically protects the programme rather than delaying it. 

“We’re not changing the existing building, so it doesn’t need looking at.” Adding storeys changes the load paths and stress levels running through the existing structure below, even where none of it is being physically altered, and under Building Regulations this can count as a material alteration in its own right. It’s also a reminder to verify rather than assume: material properties and condition in older buildings are often taken on faith rather than confirmed, and that’s exactly what a survey and appraisal are there to establish. 

Getting a Clear Answer on Your Building 

The only way to safely say whether an existing building can take additional floors is through proper investigation, not assumption. If you’re assessing feasibility for a vertical extension ahead of a planning submission, our structural engineering team can help you get a clear, evidence-based position early in the process. Contact us today by calling 0121 716 0100 or emailing mail@linkeng.co.uk.