A structural engineer answers the question of whether a building element will hold. In refurbishments, they become involved as soon as the load-bearing structure is altered: cutting through walls, creating new openings, adding storeys, heavy installations or a loft conversion. They calculate, size the structural elements and provide the documentation needed for the building permit application.
They assess the condition of the load-bearing structure, calculate loads, size beams, columns and foundations, and specify how the structure must be temporarily supported during construction. For the building permit application, they prepare the structural calculations. In older buildings, this also includes assessing the existing fabric, because historic structures cannot simply be calculated on the basis of today's assumptions.
You definitely need one whenever a load-bearing wall is opened up or removed, for additional storeys and extensions, when converting a loft into living space, for new floor openings for stairs or lifts, and for heavy installations such as a pool on the terrace. They are also the right first point of contact if cracks appear and the cause is unclear.
Clarify whether you want only an assessment or also construction drawings and site supervision. A feasibility assessment alone is significantly cheaper than a complete structural concept with drawings. Set out in writing which documents you will receive, because the building authority and the contractor carrying out the work require different documents.
For projects requiring approval, the structural engineer provides the supporting calculations that the architect incorporates into the building permit application. The earlier they are involved, the cheaper it is: a floor plan idea that does not work structurally can be changed free of charge at the beginning, but later only at considerable cost. Bring them in as early as the design stage.
Builders and carpenters work from their specifications. On larger alteration projects, site management oversees the work and checks whether the required temporary supports are actually installed. This check is not a formality, but the point where structural calculations meet site reality.
Compared with construction costs, the fee for a structural assessment looks small and is the cheapest insurance in the entire project. An unchecked opening does not show up immediately, but over months as cracks in the upper storeys, sticking doors or a sagging ceiling. Repairing such damage costs many times the original fee, and buildings insurance generally does not cover damage caused by improper structural alterations. There is also potential liability towards future buyers if an undeclared intervention in the load-bearing structure comes to light.