Most people don't ask this question early enough.
Here's a scenario we come across fairly often. Someone is deep into the planning process - their architect has done the drawings, the application is nearly ready to submit - and then someone asks: do we need a structural engineer involved at this stage?
Sometimes the answer is no, and they're fine to carry on. But sometimes the answer is yes, and the fact that nobody asked earlier has already created a problem. A structural report that was always going to be required hasn't been commissioned, and now it's sitting on the critical path.
So let's clear it up. Here's when you do and don't need structural input at planning stage. Most projects don't need it at planning.
The good news first.
For the majority of domestic projects - a standard house extension, a loft conversion, a garage conversion - planning authorities aren't interested in the structural design. They're looking at things like scale, appearance, impact on neighbours, and compliance with local planning policy. The structural side gets dealt with later, through building regulations.
So if you're planning a fairly typical residential project, you probably don't need a structural engineer involved until after you've got your planning consent. That's normal, and it's fine.
But some projects are different.
There are specific project types where structural input at planning stage isn't just helpful - it's expected, or even required. These tend to be projects where the structural approach is central to what's being proposed, rather than something that gets sorted out in the background.
Basements and underground structures
If you're proposing a basement extension - whether that's a new dig beneath an existing house or a conversion of an existing cellar - planning authorities will often want to understand the structural methodology before granting consent. How will the excavation be supported? What's the impact on neighbouring foundations? Is there a risk of ground movement? These aren't questions your architect can answer in full. You'll need a structural engineer to produce a method statement or preliminary structural report as part of the planning submission.
Heritage buildings and listed structures
Working on a listed building or a structure in a conservation area? The planning authority - and Historic England, if they're involved - will want to understand how the proposed works affect the structural integrity and character of the building. That means structural input at an early stage, often alongside a heritage consultant.
The structural report here isn't just about what's being added - it's about demonstrating that what's already there is being properly understood and protected.
Large demolitions
If your project involves significant demolition - particularly where party walls or shared structures are involved, or where the demolition affects a large or complex building - structural input at planning stage helps demonstrate that the works have been properly thought through. Some local authorities require a demolition method statement as a condition of consent.
Unusual or complex structures
Anything that's structurally unconventional - a large cantilever, a rooftop extension on an existing building, a significant change of use that affects structural loads - may need engineering input at planning to demonstrate that the proposal is structurally viable. Planning authorities aren't structural engineers, but they are entitled to ask whether something is actually buildable before they grant consent.
What does structural input at planning actually look like?
It varies depending on the project, but it's typically one of the following:
- A preliminary structural report - setting out the likely structural approach and confirming the proposal is viable
- A method statement - explaining how the works will be carried out, particularly for basements and demolitions
- A structural appraisal of the existing building - relevant for heritage projects and complex alterations
These are outline documents rather than full design packages. The detailed calculations and drawings come later, once planning consent is in place. But they need to be credible and professionally produced - a paragraph from an architect's covering letter won't cut it if the planning authority has genuine structural questions.
