Structural survey before a rooftop extension
Part 20 prior approval needs an engineer's report on whether the existing building can take additional storeys. Here are the investigations that answer it, and the service and lease issues that follow.
Before you spend anything on an upward extension, find out whether the building can carry it. Foundations sized for four storeys in 1963 were not sized for six, and the answer is a matter of measurement rather than opinion. A structural appraisal is the first cost on an airspace scheme and the only one that can stop the project outright, which makes it the best value piece of due diligence you will buy.
The permitted development route reinforces the point. Part 20 of the General Permitted Development Order 2015 requires a prior approval application for additional storeys to be accompanied by a report from an engineer or other suitably qualified professional dealing with the structural strength of the existing building and whether it can take the new loading. Read the Class that applies to your building, because the required documents and the limits differ between them.
Why capacity is tested before anything else
Three outcomes are possible and only one of them is good.
The structure has spare capacity, because it was designed conservatively or to an earlier code with generous partial factors, and the extension proceeds with local strengthening. The structure is marginal, and strengthening is possible but changes the economics. Or the structure cannot take it, and no amount of design cleverness recovers the scheme without works so extensive that demolition and rebuild becomes the comparison.
An appraisal that reaches one of those three answers in eight weeks is worth more than a planning strategy built on an assumption. And a prior approval application supported by a thin structural note is the most common reason these applications stall.
Foundations, frame and lateral stability
Four questions have to be answered with evidence.
Can the foundations take the additional load without unacceptable settlement? That means knowing the foundation type, depth and width, and the bearing stratum, which usually needs trial pits and a ground investigation rather than an assumption from the era of construction.
Can the vertical structure take the axial load? For loadbearing masonry that means unit strength, mortar strength and slenderness. For a reinforced concrete frame it means column sizes, reinforcement arrangement and concrete strength, which means coring, cover surveys and testing.
Is the building stable laterally once it is taller? Wind loading rises with height, and a building whose stability came from a few shear walls or from the stiffness of masonry cross walls may need new bracing or a new core. This is often the binding constraint rather than vertical capacity.
Will the building resist disproportionate collapse at its new height? Approved Document A sets consequence classes by building height and use, and crossing into a higher class brings tying and alternative load path requirements that an existing structure may not satisfy.
Surveys and intrusive openings
Start with the paperwork. Original drawings, calculations and the building control file for the original construction are frequently held in a local authority archive, and finding them saves weeks of destructive investigation.
Then open up. A desk study alone will not support a prior approval report.
| Investigation | What it answers |
|---|---|
| Trial pits to foundations | Type, depth, width and bearing stratum |
| Ground investigation with boreholes | Bearing capacity, groundwater, settlement risk |
| Concrete coring and cube testing | Actual strength against the design assumption |
| Cover survey and half-cell potential testing | Reinforcement position, depth and corrosion risk |
| Masonry unit and mortar sampling | Compressive strength for capacity checks |
| Cavity wall tie inspection | Whether the outer leaf is still acting as intended |
| Floor build-up exposure | Diaphragm action and the load path to the stability system |
Commission an asbestos survey before any opening up. The Control of Asbestos Regulations 2012 apply to the investigation work itself, and a block built in the 1960s or 1970s will usually have something.
Access is the practical obstacle. Opening up a floor slab in an occupied flat needs the leaseholder's cooperation and a defined scope, and the investigation programme should be planned around what access you can actually get rather than what would be ideal.
Effect on existing services and cores
An extra storey adds load to systems that were sized for the original building.
Lifts are the first problem. A lift serving four floors cannot usually be extended to six without a new shaft head, a new motor room arrangement and in many cases a new installation, and the lift overrun affects the height measurement. Risers may be full. Foul and rainwater stacks were sized for the original occupancy. Water pressure at the top floor may need a booster set. The incoming electrical supply capacity has to be checked with the distribution network operator, and the answer is sometimes a new substation.
Escape is the one to resolve first. Stair width, travel distances and the number of stairs all derive from the building as it was, and adding storeys can push the building into a different fire regime, with a fire statement required at planning stage and the whole gateway regime following behind it. Settle the fire strategy with a competent fire engineering professional before the layout is fixed, because a second stair consumes floorplate that the appraisal assumed was lettable.
Evidence for a prior approval application
The report has to do a specific job: tell the authority that the existing building is structurally sufficient for the proposed storeys, or what is needed to make it so.
A report that does that well includes the investigation record, the material test results, the load calculations for the existing and proposed conditions, the stability strategy, the strengthening works identified, and a clear statement by a named chartered engineer. A report that says the building "appears capable of supporting additional storeys subject to detailed design" does not, and an officer is entitled to treat it as insufficient.
A construction management plan goes alongside it. Part 20 expects one, and for an occupied building it is doing real work rather than ticking a box, so the content set out for a construction management plan should be drafted around the fact that residents stay in place throughout.
Keep the whole evidence pack together for warranty and sale purposes later, as the argument in structural warranties and what the planning file should hold explains.
Neighbour and leaseholder issues
For a block of flats, the airspace is usually retained by the freeholder, but the lease is what decides it. Read the demise, the common parts definition and any rights granted over the roof before assuming you own what you intend to build on.
Two statutory points need legal advice rather than a rule of thumb. A disposal of airspace by the landlord can engage the right of first refusal under the Landlord and Tenant Act 1987, and a sale of the airspace to a developer without following that procedure creates a serious problem. And where the works affect a party structure, the Party Wall etc. Act 1996 procedure applies with its own notices and timescales.
Leaseholders will also raise the practical objections: noise, loss of light, scaffolding outside windows, loss of roof terrace access, lift outages and the service charge consequences of the strengthening works. Those are not all planning matters, but they shape whether the project is deliverable.
Programme and access constraints
Illustrative example, not a real scheme: a five storey purpose-built block of 20 flats, built in the early 1970s, is appraised for two additional storeys. The desk study finds the original calculations in the council archive, trial pits confirm pad foundations on dense sand with capacity in reserve, concrete coring returns strengths above the design assumption, but the stability analysis shows the shear walls cannot take the increased wind load at seven storeys. The scheme proceeds at one additional storey instead of two, with localised strengthening and no new core. Eight weeks of investigation changed the scheme from unviable to viable by changing its size.
Plan the construction logistics at the same time. Crane or hoist standing, scaffolding that may need a licence or a neighbour's permission, temporary weatherproofing over occupied flats, working hours on a residential street and the sequencing of lift works all affect cost more than the structure does.
The same discipline applies at the smaller end. On a house rather than a block, the governing constraints are different but the method is identical, as set out in structural limits on a terraced house loft conversion.
But commission the structural survey first. Everything else is a guess until it lands.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Structural appraisal and fire engineering require suitably qualified professionals, and councils usually expect a suitably qualified author, so professional review is strongly advised.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Does an upward extension need a structural report?
Yes, where the prior approval route under Part 20 of the General Permitted Development Order 2015 is used. The application has to be accompanied by a report from an engineer or other suitably qualified professional dealing with the structural strength of the existing building and whether it can take the new loading. The required documents differ between the Classes, so read the one that applies.
What usually stops a rooftop extension?
Lateral stability more often than vertical capacity. Wind loading rises with height, and buildings whose stability came from a small number of shear walls or from masonry cross walls frequently cannot take the increased load at the new height without new bracing or a new core. Foundations and column capacity are often found to have reserve.
What intrusive investigation is needed?
Trial pits to the foundations, a ground investigation for bearing capacity and groundwater, concrete coring and testing, a cover survey and half-cell potential testing for reinforcement, masonry and mortar sampling, cavity wall tie inspection, and floor build-up exposure to confirm the load path. Commission an asbestos survey before any of it, since the regulations apply to the investigation itself.
Do I need leaseholder consent?
Read the lease first, because the demise and the common parts definition decide who controls the roof and the airspace. A disposal of airspace can engage the right of first refusal under the Landlord and Tenant Act 1987, and works affecting a party structure bring the Party Wall etc. Act 1996 procedure into play. Both need legal advice rather than a rule of thumb.
How do existing services affect feasibility?
Considerably. Lifts serving the original floors usually cannot simply be extended, risers may be full, foul and rainwater stacks were sized for the original occupancy, top floor water pressure may need a booster, and the incoming electrical capacity has to be checked with the distribution network operator. Escape stair capacity should be resolved before the layout is fixed.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.