Plenty of buildings already have a full set of 2D CAD drawings — plans, sections, elevations — that were perfectly adequate when they were produced. As more projects move to BIM-based workflows, though, those flat drawings become a limitation: they don’t hold the spatial and data relationships a 3D model does, and they’re only as accurate as whatever survey or record they were originally based on. CAD to BIM conversion turns existing 2D drawings into a working 3D model, but it’s not automatically the right move for every set of drawings.
For architects, developers and facilities managers across Hertfordshire, Essex, East Anglia and Cambridgeshire deciding whether to convert existing drawings, here’s what the process actually involves and when it’s worth doing.
From flat CAD lines to a full 3D model — what upgrading old drawings to BIM actually reveals.
What CAD to BIM conversion actually does
The process takes existing 2D CAD drawings and rebuilds them as a 3D BIM model — walls, floors and structural elements modelled with real dimensions and relationships, rather than represented as flat lines. Where the original CAD drawings are accurate and complete, this can be a relatively straightforward modelling exercise. Where they’re incomplete, outdated, or were never verified against the actual building, conversion on its own simply carries those inaccuracies into a more sophisticated-looking format.
The critical question: how accurate are your existing drawings?
This is the point most conversions get wrong. CAD to BIM conversion models what’s on the drawing — it doesn’t verify that the drawing matches the building. If the original CAD was produced from an accurate measured survey and has been kept up to date through subsequent alterations, conversion is a reasonable, cost-effective route to a usable BIM model. If the drawings are old, were never fully accurate, or the building has been altered since without the drawings being updated, converting them into BIM produces a confident-looking model that’s still wrong — arguably a worse outcome than staying in 2D, because the format now implies an accuracy the data doesn’t have.
When CAD to BIM conversion makes sense
Conversion is the right call when existing drawings are known to be accurate and reasonably current, when the project needs a 3D model primarily for coordination or visualisation rather than construction-stage precision, and when budget doesn’t stretch to a full re-survey but a workable 3D model is still needed. It’s a genuinely useful, lower-cost route to BIM in the right circumstances.
When a fresh scan is the better investment
Where drawings are of uncertain accuracy, where the building has had undocumented alterations, or where the model will be used for anything requiring genuine as-built precision — construction coordination, clash detection, retrofit design — a fresh 3D laser scan and scan-to-BIM process is the more reliable route. It’s a larger initial investment than converting existing drawings, but it avoids building a project’s design decisions on an assumption that may not hold.
How to decide which route is right
Before committing to either route, it’s worth having someone independently assess how reliable your existing CAD drawings actually are — when they were produced, whether they were based on a proper survey, and whether the building has changed since. That assessment alone often settles the question of whether conversion or a fresh scan is the better investment.
Working with AKN Engineering
AKN Engineering provides CAD to BIM conversion, and where appropriate a fresh 3D scan and scan-to-BIM alternative, for architects, developers and facilities managers across Hertfordshire, Essex, East Anglia and Cambridgeshire — helping you choose the right route before committing budget to either.
If you have existing CAD drawings and are considering a move to BIM, speak to AKN Engineering on 01279 927 033 or visit www.aknengineering.co.uk.