Structural engineers for basement and retrofit work

For a domestic basement or a structural retrofit, the engineer you want is a chartered member of the Institution of Structural Engineers or a chartered civil engineer with a structural practice, and you need to check one thing beyond the letters: whether they will also act as temporary works designer, or whether that is a separate appointment the contractor has to make. Structural engineer basement qualifications are not the hard part. The gap between permanent works design and temporary works design is where domestic basement projects go wrong, and it is almost never explained to the homeowner.

Underpinning a party wall is temporary works until the new structure is complete. Someone has to design the sequence.

Chartered routes and what each covers

Two chartered routes matter for this work, and a third is adjacent.

A Chartered Structural Engineer, MIStructE, has passed the Institution of Structural Engineers chartered membership examination, which is a design examination. For basements and retrofit that is the most directly relevant qualification. A Chartered Civil Engineer, MICE, may be an excellent structural designer or may specialise in highways, water or geotechnics, so the title alone does not tell you. A Chartered Building Surveyor, MRICS, can diagnose defects and specify repairs but is not a structural designer and should not be asked to produce underpinning calculations.

Ask directly: are you chartered, with which institution, and how many domestic basement or underpinning schemes have you designed in the last two years. Then ask about professional indemnity insurance, the level of cover and whether it is held on a claims-made basis, because a basement is a long-tail risk and a lapsed policy is worth nothing when a crack appears in year six.

An engineer practising without chartered status is not automatically unsuitable, and some very experienced designers are incorporated rather than chartered. Insurance and a track record on comparable work matter more than the postnominals on their own.

Basement and retrofit specialism against general practice

A general structural engineer designing a steel beam for a knocked-through wall is doing routine work. A basement under an existing house is not routine.

The specialist knowledge is in four places. Ground and groundwater behaviour, which means reading the site investigation rather than taking it on trust. Movement prediction, because the question the party wall surveyor will ask is how much the neighbour's wall will move and when. Waterproofing design to BS 8102, which is a design discipline with three types of protection and a drained cavity decision that the structural engineer usually has to coordinate even where a specialist waterproofing designer is appointed. And sequencing, which is the temporary works question.

For retrofit, the specialism is different again: existing material properties, lime mortar and solid wall behaviour, hidden alterations by previous owners, and the interaction between structural work and moisture performance in a traditionally built house. An engineer who designs new-build frames all week may be entirely unfamiliar with a 1890s terrace that has already been altered three times.

Ask for two previous schemes of the same type, with the ground conditions named. Not a list of project names.

Temporary works design as a separate appointment

This is the thing to settle before any fee is agreed.

Permanent works design is what the finished structure has to be. Temporary works design is how you get there without the building falling down in the meantime: the propping scheme, the underpinning sequence, the pin widths and the hit-and-miss order, the excavation support, the needling and the shoring. Under the Construction (Design and Management) Regulations 2015 the designer has duties, and the contractor has duties for the work itself, with a temporary works coordinator role in the recognised arrangements for managing it.

On a domestic basement the common arrangement is that the permanent works engineer designs the permanent structure and the specialist contractor designs the temporary works. That can work well, because the contractor owns the sequence they will actually build. It fails when nobody says out loud who is doing it, the homeowner assumes the engineer has covered it, the contractor assumes the drawings they were given are a sequence, and excavation starts against an unpropped party wall.

So ask three questions at appointment. Who designs the temporary works. Who checks that design, and to what category. Who is the temporary works coordinator on site. Get the answers in the appointment letter.

Calculations the building control body will want

Building control is separate from planning permission and it is where the structural submission is examined.

Expect to provide design calculations for the retaining structure with the soil parameters and surcharges used, foundation and bearing capacity checks against the site investigation, calculations for the underpinning sequence and pin design, a waterproofing design statement to BS 8102 naming the grade of internal environment, drainage and sump arrangements with a plan for maintenance and power supply, and a movement assessment for adjoining structures. Where excavation goes deep or the ground is difficult, a Geotechnical Assessment prepared to Eurocode 7 sits behind the calculations.

Where the planning application itself requires a Basement Impact Assessment, as many London boroughs do, that is a planning document about groundwater, hydrology, ground movement and the cumulative effect on neighbours, and it is not a substitute for the building control calculations. Two documents, two audiences. A Structural Survey of the existing house usually comes first and establishes what you are building onto.

Working with the party wall surveyor

The Party Wall etc. Act 1996 process runs in parallel and the engineer's drawings feed it.

The adjoining owner's surveyor will scrutinise the movement prediction, the underpinning sequence, the monitoring regime and the proposed trigger levels. An engineer who has done this before will produce a movement assessment in a form the surveyor recognises, with predicted categories of damage, and will propose a monitoring scheme with defined trigger and action levels before being asked. An engineer who has not will produce a letter saying movement is expected to be negligible, and the award will be delayed while the adjoining surveyor appoints their own engineer at your cost.

Monitoring is not optional on a basement adjoining another property. Levelling stations, crack monitors or precise targets, read at stated intervals from a baseline taken before work starts, with a written response if a trigger is reached.

Site inspection duties during construction

Design without inspection is a drawing. Agree the inspection regime in writing.

What to specify: inspection at each defined hold point, which for a basement means before concreting each underpin sequence, at completion of the retaining wall, at waterproofing installation and before covering up, and at completion of the drainage and sump. Agree who issues the inspection record, how quickly, and what happens if the work found does not match the design.

Ask whether inspections are included in the fee or charged per visit, because a basement can run to a dozen visits and the difference between included and additional is substantial. Also agree who answers the contractor's queries during construction and within what time, since an unanswered query on site becomes a decision made by the groundworker.

Agreeing scope before the fee

Write the scope down before comparing prices, because the quotations will otherwise be for different work.

The scope should state the permanent works design, whether temporary works design is included or excluded, the waterproofing design responsibility, the movement assessment and monitoring specification, the party wall support including attendance at meetings and correspondence with the adjoining surveyor, the building control submission and responses to queries, the number of site inspections included and the rate beyond that, and the handover deliverables including as-built information and the maintenance requirements for the drainage system.

Then compare. A fee that is a third of the others is usually a permanent works design with everything else excluded, which is a legitimate offer and a different one. The same reasoning applies to choosing any specialist author, and checking a consultant's appeal track record is the equivalent test where planning evidence is involved, while changing consultant during a live planning application covers the handover risks if an appointment goes wrong.

This is general guidance, not legal advice; check the position with your local planning authority, your building control body or a qualified professional before relying on it. A structural survey and a basement impact assessment are reports councils expect a suitably qualified author to prepare, and professional review is strongly advised.