LIBRARY // ACCURATE MODELLING AND RESEARCH

Converting Legacy CAD Drawings to BIM

Old CAD drawings hold a lot of useful information, but turning them into a reliable model needs care.

Many building owners have CAD drawings of their existing buildings but no BIM models. Converting these drawings into BIM can support refurbishment, facility management and future projects. The result is only as reliable as the source drawings, so the process needs care.

Check the source drawings

Before modelling:

• Check drawings are complete
• Check consistency between plans, sections and elevations
• Identify the drawing date and revision
• Check whether drawings reflect the current building
• Note missing information

Verify against the building

Where possible, check key dimensions on site or against surveys. If drawings are unreliable, a survey may be needed.

Set up coordinates

Agree a coordinate system and level datum before modelling. Align CAD drawings to this system when importing or linking them.

Linking versus importing

In Revit, linking CAD files keeps models lighter and easier to update, while importing embeds them. Linking is generally recommended for reference.

Modelling strategy

• Set up levels and grids first
• Model structure, then walls, floors and roofs
• Add doors, windows and openings
• Add rooms and areas
• Add other elements as needed

Model to the agreed level of development, not more.

Resolving inconsistencies

When drawings disagree, decide which source to trust, or check on site. Record decisions.

Assumptions

Some information won't be available, such as wall build-ups or hidden structure. Make reasonable assumptions, mark them clearly and record them in a log.

Data

Add data needed for the model's purpose, such as room names, areas or asset information for facility management.

Quality checks

• Overlay the model on CAD drawings
• Check areas against known values
• Review warnings
• Check views and sheets

Use of the model

Documentation

Provide a short report explaining sources, assumptions and accuracy. It helps future users.

Check against reality

Legacy drawings may not reflect later alterations. Where possible, compare the drawings with a site visit or a quick survey of key dimensions before modelling, and note which areas were verified.

1. 01Check the source drawings
2. 02Verify against the building
3. 03Set up coordinates
4. 04Linking versus importing
5. 05Modelling strategy
6. 06Resolving inconsistencies

Plan the effort

Converting large sets of drawings takes time. Prioritise the areas the project actually needs, rather than modelling the whole building in detail.

Converting legacy CAD to BIM can unlock value from existing information when sources are checked and assumptions recorded.

If your team needs extra production capacity for work like this, take a look at our Architectural production services, or tell us about your project.