Architect vs Structural Engineer: Who Do You Actually Need?

Search this question and you'll find forum threads full of people who hired one, then found out halfway through they needed the other. Here's the honest version, including where an architectural designer fits in — because that's what most practices doing home extensions in the UK actually are, including us.

Three different jobs

An architect or architectural designer designs the space. What goes where, how the light works, how the new part sits with the old, how it looks from the street, and — critically — how it gets through planning. They produce the survey, the concept, the planning drawings, and usually the building regulations drawings too.

A structural engineer works out how it stands up. Which walls are load-bearing, what size of steel beam goes in when you knock through, whether the existing foundations can take a second storey, how a loft floor is strengthened. They produce calculations and structural details that building control checks before signing the work off.

The difference between an architect and an architectural designer is a legal one, not a practical one for this kind of work. "Architect" is a protected title in the UK, reserved for people registered with the Architects Registration Board. An architectural designer does the same residential design work without being on that register. Diogel are architectural designers, not architects, and we say so plainly because the distinction matters and we'd rather you heard it from us.

When you need which

For almost any home extension or loft conversion, you need both a designer and an engineer. Here's why:

• Rear or side extension: knocking through into the existing house nearly always means removing part of a load-bearing wall and installing a steel. That needs an engineer's calculation, full stop. The design of the space itself — the bit you'll live in — is the designer's job.
• Loft conversion: the existing ceiling joists are almost never strong enough to be a floor. New floor joists, steel beams, strengthening the roof if you're adding a dormer — all engineer territory. The layout, the stairs, the head height and light are the designer's.
• Two-storey extension: foundations, whether the existing ones can take the load, how the new upper floor ties in. Engineer. Everything about how the rooms work: designer.
• Garage conversion (as-is, no enlargement): sometimes the lightest structural involvement — but if you're opening up the front, the lintel over the new opening still usually needs a calculation.

The one case where you can genuinely go engineer-only is simple like-for-like work with no design decisions to make — replacing a beam, say. If you're changing how you live in the house, you want a designer.

Do you have to hire them separately?

Not usually, and you shouldn't want to. The engineer's work has to match the design, and the design sometimes needs to move because of what the engineer finds — a wall that turns out to be doing more than expected, foundations shallower than hoped. If you've hired the two independently, you become the messenger between them.

The better model is the one we use: the design practice brings the structural engineer in at the building regulations stage, briefs them, and folds their calculations into the drawings your builder works from. You deal with one team. The engineer's fee is a separate line, but it's coordinated, not something you have to go and source yourself.

What about cost?

Two honest points rather than a fabricated table:

1. The structural engineer is a small slice of the total. For a typical extension or loft it's a few hundred to low thousands of pounds, depending on complexity — a fraction of the build cost, and not somewhere to economise.
2. Our design fee is fixed, not a percentage of the build. We agree it before any work starts. That's worth knowing because some practices charge a percentage, which means the more your project costs, the more you pay them for the same drawings.

The short version

You need…	For…
An architectural designer (or architect)	Designing the space, the planning application, building regs drawings, dealing with the council
A structural engineer	Beams, foundations, load-bearing walls, structural calculations for building control
Both, coordinated by the designer	Almost every extension and loft conversion

If you're not sure which camp your project falls into, that's exactly what a free call is for — we'll tell you honestly what it needs, including if it needs less than you thought.