Knocking through a kitchen and dining room. Widening a doorway at the back of the house. Adding a dormer so the loft finally earns its keep. Or terracing a steep garden so you can actually sit out on a summer evening without clinging to the fence.

These projects look straightforward on paper. Then a builder lifts a floorboard, frowns at what's underneath, and asks the question you were hoping to avoid: "Have you had a structural engineer look at this?"

If the answer is no, work usually stops. Hiring a structural engineer early is one of the least glamorous parts of a renovation, but it is often the difference between a smooth build and a costly, stressful mess.

What a Structural Engineer Actually Does

A structural engineer works out how a building carries its loads and how it will behave once you start changing things. On a domestic renovation, that means three main jobs:

• Identifying load-bearing elements. Not every wall is structural, and not every wall that looks flimsy is safe to remove. Engineers assess walls, floors, roofs, chimneys and existing beams to establish what is holding what up.
• Specifying safe alterations. If a wall is load-bearing, they calculate the steel beam or lintel required, the padstones it sits on, and the fixings and bearings needed at each end.
• Providing calculations and drawings. Building control will want evidence that your proposals are safe. Your engineer supplies the calculations, details and specifications that satisfy them.

They are not the same as an architect. An architect designs the space and how it looks; a structural engineer makes sure it stands up. On simple jobs one person may cover both, but on anything involving removed walls, extra storeys or significant groundworks, you want a proper structural input.

When You Definitely Need One

Some projects are a clear yes. Book an engineer before you instruct a builder or order materials if your plans involve any of the following:

• Removing or altering any wall you are not certain is non-structural
• Creating an open-plan kitchen, dining or living space
• Adding a loft conversion, dormer or roof lights that affect the roof structure
• Building a single or two-storey extension, or a garage conversion
• Underpinning, basement work or significant excavation
• Removing a chimney breast, above or below
• Installing a large sliding or bifold door where a wall used to be
• Any work on a period property, where lintels, timber and previous alterations may be a lottery

If you are simply replacing a kitchen, repainting, or moving a stud partition that a surveyor has confirmed is non-load-bearing, you can usually skip it. When in doubt, ask. A short consultation costs far less than remedial work.

Why Early Involvement Saves Money

The single biggest mistake homeowners make is hiring an engineer after the design is fixed and the builder is booked. At that point, the engineer's job becomes telling you what you cannot do rather than helping you find what you can.

Bring them in at design stage and they can suggest alternatives. Perhaps a steel frame lets you remove more of a wall than you expected. Perhaps a flush beam hidden in the ceiling void avoids a dropped soffit. Perhaps the garden levels can be terraced with a retaining wall design that uses less concrete and fewer lorries.

They can also flag problems before they become expensive: shallow foundations where an extension is planned, sagging floor joists that need strengthening, or a party wall issue that needs resolving with a neighbour. Sorting these during design is cheap. Sorting them mid-build is not.

Sloped Gardens, Retaining Walls and Ground Movement

Sloped gardens are where structural engineering gets genuinely interesting. Any time you hold back soil at a different level, you need a retaining wall, and retaining walls fail more often than most homeowners realise.

An engineer will consider the height of the retained soil, the type of ground, drainage, and whether the wall needs to resist water pressure as well as earth. A wall one metre high is a different proposition from one two metres high, and the consequences of getting it wrong are serious, especially if it sits near a house, boundary or public path.

For terraced gardens, they will also advise on steps, levels, and how new structures interact with existing foundations. Excavating near a house can reduce the support beneath it, so the sequence and distance of any digging matters. If you plan to add a deck, cabin or seating area into a slope, have the retaining structure designed properly rather than trusting a rule of thumb.

How to Hire the Right Engineer

Look for a chartered engineer, ideally a member of a recognised professional institution, with experience of domestic work rather than only commercial projects. Ask for examples of similar jobs: loft conversions, open-plan knock-throughs, garden retaining walls.

Get a written fee proposal that states what is included: site visit, calculations, drawings, and how many revisions. Some engineers will liaise directly with building control and your builder, which is worth paying a little extra for.

Expect to pay a few hundred pounds for a simple beam calculation and nearer a thousand or more for more complex work, depending on your area and the scope. Always check they hold professional indemnity insurance.

Finally, keep your engineer in the loop during the build. If your builder finds something unexpected, a quick call is far better than a guess. Good structural advice is not an obstacle to your renovation; it is what lets you build with confidence, open up the space you have always wanted, and enjoy a garden that works with its slope rather than against it.