An architect decides where the wall goes. A structural engineer works out what holds the house up once it is gone. Most extension projects need both, and the confusion about which one to call first costs people money. Anyone searching for a structural engineer near me before there is a design to calculate is usually calling in the wrong order.
Here is what each role actually covers, where they overlap, and when each one needs to be involved.
What the Architect Does
The architect handles design and consent. That means establishing what the plot and the planning rules allow, then designing a scheme that works within those constraints.
Practically, that covers the measured survey of the existing building, the planning application drawings, the heritage statement where a conservation area is involved, and the building regulations drawing package. On a full service appointment it also covers tendering the work to builders and administering the building contract during construction.
Architects registered with the ARB are the only people entitled to use the title in the UK, and RIBA chartered practices work to an additional set of professional standards. Both are worth checking.
The design decisions the architect owns are the ones that shape how the space feels. Where daylight comes from. How deep the extension can go before the middle of the plan goes dark. Whether the threshold to the garden sits flush or steps down. What the roof does where it meets the original building.
What the Structural Engineer Does
The engineer handles load. Their job is to work out what the building is currently carrying, where that load travels, and what has to replace anything you remove.
The output is a calculation pack. Load calculations showing what goes where. Beam sizing and specification, giving the section, grade and required bearing length. Foundation design where new foundations are involved. Connection details where new structure meets old. And structural drawings the builder works from.
Building Control assesses that pack before signing off structural work. Without it, the work does not get approved, and unapproved structural work becomes a problem at sale.
Look for chartered status through the Institution of Structural Engineers or the Institution of Civil Engineers. The letters are MIStructE, CEng or MICE. Professional indemnity insurance sits behind the work, and if a design ever fails, the calculations are the document everyone returns to.
Where the Two Overlap
The overlap is the interesting part, and it is where projects go wrong when the two are appointed separately and sequentially.
Beam span drives cost, and beam span is a design decision. Removing an entire rear wall needs a considerably larger steel section than a design retaining a pier in the middle. That difference shows up in the steel cost, in whether a crane is needed, and sometimes in the foundation design at the bearing points.
So the question of where the wall opening sits is simultaneously an architectural decision about how the room feels and an engineering decision about what it costs. Resolved together in week two, it is free. Resolved after planning permission, you either absorb the cost or apply to amend an approved scheme.
The same applies to foundations. Standard foundations run around a metre deep, but in clay soils that can rise to 1.5 metres or more, and a mature tree within influencing distance changes the required depth substantially. Whether the extension footprint can avoid the tree is a design question with an engineering answer.
The Order That Avoids Rework
Most people appoint an architect, get planning permission, then look for an engineer. That works, but it is not the cheapest sequence.
A better order looks like this.
Survey the existing structure first, including foundation depth and party wall construction. On period properties this matters, because Victorian houses frequently sit on foundations 400 to 600 millimetres deep and half brick party walls are common. Both change the structural approach entirely, and both are found by survey rather than assumption.
Establish the planning position before design. Conservation area status and any Article 4 direction on the exact address, plus the planning history through the council portal, since previous permissions sometimes carry conditions removing permitted development rights.
Design the scheme with the structural strategy considered rather than assumed. This is the step that gets skipped.
Get calculations before finalising drawings, not after.
Serve party wall notices early. Cutting into a shared wall triggers notice under Section 3 of the Party Wall Act 1996, and excavating near a neighbour’s foundation triggers Section 6. Two months notice for Section 3, one month for Section 6.
Then tender. A builder pricing a scheme with calculations already done is pricing something real. A builder pricing without them is pricing an assumption, and the gap between those two numbers reappears later as a variation.
Where Building Control Fits
Neither the architect nor the engineer approves the work. Building Control does, and you have a choice of route.
The local authority route goes through the council’s own Building Control department. The alternative is an approved inspector, a private body carrying out the same statutory function.
Approved inspectors are often more flexible on site visit timing. Local authority Building Control has direct access to the council’s records on the property. Either is valid, and the choice is worth making early rather than during the build.
Why Some Practices Combine the Two
Most architectural practices outsource structural work. The design goes out, calculations come back, and any conflict between what was drawn and what is structurally sensible gets resolved late.
Extension Architecture keeps a structural engineer within the same team as the design staff. The practical effect is that beam spans get tested while the layout is still fluid, rather than after a client has committed to a plan.
It removes a handoff, and handoffs are where programmes lose weeks.
What This Looks Like on a Wimbledon Project
South west London has a mixed housing stock, and the structural questions vary considerably with it.
On a Victorian or Edwardian terrace, the recurring issues are shallow foundations, half brick party walls, and roof structures that were never designed to carry a floor. The engineer’s involvement is heaviest at survey stage.
On the interwar and postwar detached and semi detached stock, plots tend to be larger and foundations more predictable, but mature trees are common and clay conditions across parts of the borough affect foundation depth. Here the engineer’s role weights towards foundation design.
Either way, the local planning position shapes what gets designed before either professional starts. architects wimbledon homeowners appoint should be confirming conservation area status and Article 4 directions on the specific address at the outset, because the answer determines whether you are designing to a table of permitted development limits or building a case for an officer.
The Short Answer
Call the architect first if you do not yet know what you are building or whether it is allowed.
Call the engineer first only if you have a specific structural question about an existing building, such as whether a wall is load bearing or whether a crack is a concern.
For an extension, it is almost always the architect first, with the engineer brought in during design rather than after it.
