Can you create a digital twin for an existing building without BIM?

Yes. A useful operational digital twin can be built for a defined part of an existing building without first reconstructing a detailed whole-building BIM model. What a twin needs depends on the decision it supports, and for most operational questions a verified record of the relevant equipment, the spaces it serves, how it is connected and the data it produces matters far more than detailed geometry. Commissioning a full 3D model as the first step often consumes the budget that verification and integration needed.

The real difficulty in an existing building lies elsewhere. Its records are often incomplete and partly wrong, and the work is in assembling a foundation that can be trusted.

What a twin actually needs

At its simplest, a digital twin is a digital representation of something real. For an existing building, the practical question is what to represent, and how faithfully. In an existing building it helps to think in layers:

Spatial tasks, such as planning a plant replacement route, coordinating a fit-out or visualising complex risers, can justify geometry. Diagnostic and energy tasks usually do not, at least at first.

Old records are only evidence

Drawings, schematics, asset registers and BMS point lists in an older building describe what someone intended or recorded at some point in the past. For the chosen scope, the work is to gather everything available, from drawings and schematics to BMS graphics, point lists, the asset register and maintenance history, and then to test it on site by confirming equipment, nameplates, labels and locations and tracing the key connections. Where sources disagree, the team reconciles them and records which version was confirmed.

The final step is recording uncertainty, and it is frequently left out. Each relationship should be marked as verified, inferred or unknown, so that users know what they can rely on.

Identities first, then connections

Each asset needs a stable identifier, ideally matching a physical label, used consistently in the asset register, the maintenance system and the data layer. BMS points and meters are then mapped to those assets and spaces; an ontology such as Brick can hold the relationships in a documented, reusable form. A limited, verified scope makes a stronger foundation than a comprehensive model assembled from conflicting records.

None of this depends on BIM having been used to deliver the building. ISO 19650-3, which covers information management in the operational phase, applies to all types of assets.

Brief the survey like a construction package

A survey for an existing-building twin is an information-gathering contract, and its brief should be as precise as any construction package. It should cover:

• the scope boundary: the systems, plant rooms and spaces included, and what is explicitly excluded;
• the verification standard: what counts as "verified" (a nameplate photograph, a route traced on site, a point confirmed by switching equipment) and what is recorded as "inferred";
• the deliverables: an asset list with identifiers and label positions, a relationship list with its confidence status, marked-up schematics, a point-to-asset mapping, a photographic record and a discrepancy log;
• the format: structured files that load into the asset register and data layer, not only a PDF report;
• access and operating constraints: occupied areas, permits, out-of-hours working, and any tracing that requires a system to be switched or isolated, agreed with the operator in advance;
• who decides when the drawings, the BMS and the site disagree.

When a laser scan is worth it

A laser scan or other measured survey captures geometry quickly and accurately, and it is worth commissioning for spatial tasks: a congested plant room, a riser, a replacement route for large equipment. What it does not record is what an asset is, who maintains it or what it serves. Those still come from the asset survey, so a scan supplements that work; it does not replace it.

Keep the twin true after the surveyors leave

Someone has to update the representation when equipment is replaced, circuits are altered or spaces change use, and someone has to verify those updates. Without that arrangement, the survey team's work begins to decay as soon as they leave site.