Scan-to-BIM for a Grade II listed office retrofit in London

Retrofit projects on listed buildings present a specific challenge for as-existing surveys: the assumption that walls are plumb, floors are level, and angles are orthogonal does not hold. In a building with 150 years of incremental alteration, the actual geometry can diverge from the architectural drawings by tens of millimetres in ways that have material consequences for services routing and fit-out tolerances.

We were commissioned to provide a point cloud survey and as-existing BIM model for a seven-storey Grade II listed office building ahead of a full internal retrofit. The existing drawings dated from a 1990s refurbishment and had not been updated since.

The survey methodology was agreed with the lead architect before scanning commenced: a tolerance specification of ±5mm across all floors, with LOD 300 modelling of all structural elements and major MEP services, and LOD 200 modelling of existing fit-out to be stripped.

Scanning was conducted across seven floors over three days, with 340 scanner positions to achieve the required registration accuracy. The point cloud was processed and the registration error verified before any modelling commenced.

The as-existing model was delivered six weeks after survey completion. The design team confirmed at their first Stage 3 coordination meeting that the model had eliminated all survey-related queries that had been anticipated based on the existing drawings.