Scan to BIM
Scan to BIM for Existing Buildings:
How to Brief It So the Model
Is Actually Usable
A point cloud shows what is there. A BIM model is what your team will design, coordinate and quote from. The gap between the two is decided by the brief you write before modeling starts.
Key takeaways
- Most scan-to-BIM disappointment comes from a vague brief, not from the scan. Say what the model is for before you say how detailed it should be.
- Level of Development (LOD) and Level of Accuracy (LOA) answer different questions: how much is modeled, and how closely it matches the building.
- A scan cannot see inside walls, above ceilings or below floors. Decide up front how those areas are treated and labeled.
- Ask for a model that is checked against the scan, with queries listed rather than guessed.
Why existing buildings need a proper brief
Designing for a building that already exists starts with a question new-build teams never ask: what is actually there? Old drawings are often incomplete or out of date, and walls, floors and services rarely match the originals after years of alterations.
That matters because so much work now happens in existing buildings. In the United States, the American Institute of Architects found that renovation work made up 52% of architecture firm billings in 2022, the first time it passed 50% in the 20 years of data AIA had tracked. It was about one-third in 2005 and 44.4% in 2017.
US figures from AIA architecture firm billings, reported by Building Design + Construction in November 2022 (a secondary source). They measure firm billings, not total construction spending, and they cover US urban markets, so treat them as a direction of travel rather than a global number.
For a team in this position, a scan-based model is a practical way to start from measured conditions instead of assumptions. It only works if the model is built to a clear specification.
Scan to BIM in plain terms
Laser scanning (or photogrammetry) captures millions of measured points on the surfaces of a building. Scan-to-BIM is the modeling step that follows: walls, floors, openings, structure and services are built as intelligent objects, usually in Revit, on top of that point cloud. Our guide to point cloud modeling and conversion covers the data side for surveyors and engineers. This article is for the people who will use the model: architects, owners, contractors and project managers.
Start with the purpose of the model
| If the model is for… | It probably needs | Watch out for |
|---|---|---|
| Feasibility or space planning | Walls, floors, openings and room areas in good geometry | Spending on detail nobody will use |
| Renovation design | Structure, envelope, floor levels, openings and key fixed elements | Floor-to-floor heights and slab edges that drive the design |
| MEP retrofit and coordination | Located ducts, pipes, trays and equipment with sizes | Services hidden behind ceilings or inside shafts |
| Facade or envelope work | Openings, reveals, parapets and elevation geometry | Access limits for scanning high or inaccessible areas |
| Facilities and asset records | Rooms, equipment and tagged data attached to objects | Inconsistent naming and missing parameters |
Write the purpose into the brief. A model for space planning and a model for MEP coordination can start from the same scan, but they are different jobs.
LOD and LOA: two different questions
LOD: how much is modeled
The BIMForum Level of Development Specification (2025 edition) defines LOD 100 to 500. For existing buildings, LOD 500 is described as a graphic representation of an existing or as-constructed condition developed through a combination of observation, field verification or interpolation, and the accuracy level must be noted or attached to the element.
LOA: how closely it matches
The USIBD Level of Accuracy Specification sets out a standardized framework for defining and validating the accuracy of building documentation deliverables. Version 3.1 was released in January 2025; the original dates from 2014.
Two points from the LOD document are worth knowing. First, it says LOD 500 does not indicate a higher level than LOD 400; it describes geometry taken from an existing item rather than designed. Second, it states that LOD concerns how far project team members may rely on the information in the model, not simply how much detail it has. It also says it does not prescribe which LOD should be reached at which stage of a project, so that decision belongs in your brief.
Both specifications are detailed documents. Read them, or ask your consultant to, before you cite specific levels in a contract. The summary above is not a substitute for the documents.
What a good scan-to-BIM brief contains
Purpose and users
What decisions the model supports and who will open it: designers, contractors, the owner.
Scope by element
Which elements are modeled (structure, envelope, partitions, MEP, furniture) and which are left out.
Accuracy requirement
The tolerance you need and where, ideally expressed through an accuracy standard such as USIBD LOA.
Hidden and inaccessible areas
How areas the scan could not see will be treated, labeled or excluded.
Software and standards
Revit version, template, naming, parameters, levels and grids, and the coordinate system or survey control.
Deliverables
Native model, IFC if needed, plans and sections, a deviation or QA report and a list of open queries.
What a scan cannot tell you
- Concealed construction: structure, services and cavities behind finishes are not visible to the scanner.
- Materials and condition: a point cloud shows geometry, not what a wall is made of or how sound it is.
- Occluded areas: furniture, stored items and tight spaces leave gaps in the data.
- Services above ceilings: unless ceilings were open or access panels removed during the scan.
The practical answer is to label these areas honestly. A model element that is assumed, inferred or interpolated should be marked as such, not shown with the same confidence as a surface the scanner measured directly.
How a scan-to-BIM project typically runs
- Brief and scopeAgree the purpose, elements, accuracy requirement, software and deliverables before modeling begins.
- Data reviewThe point cloud is checked for coverage, registration quality and missing areas, so gaps are known before they become modeling guesses.
- Levels, grids and setupThe Revit project is set up with correct levels, grids and coordinates taken from the scan.
- ModelingStructure and architecture are modeled first, followed by MEP and other elements in scope.
- Checking against the scanThe model is overlaid on the point cloud to find deviations. Items that cannot be resolved from the data are listed as queries.
- Review and handoverYou review the model and query list. Corrections are made and the model is delivered in the agreed formats.
Common problems, and how they are handled
| Problem | How it is handled |
|---|---|
| Walls that are not plumb or flat | Agree in the brief whether to model ideal geometry or as-measured geometry, and where the tolerance sits. |
| Sloping floors and uneven slabs | Set levels deliberately and document the variation instead of hiding it. |
| Curved or irregular elements | Choose between a close approximation and a more detailed element depending on purpose and cost. |
| Gaps in the scan | List them, mark affected elements as inferred, and re-scan if the area is critical. |
| Services partly visible | Model what is seen, mark sizes and routes that are assumed, and flag them for site verification. |
| Model disagrees with old drawings | Treat the scan as the evidence, report the difference, and let the design team decide. |
The common thread is that an uncertain element should be flagged, not smoothed over. A model with an honest query list is easier to trust than one that looks perfect.
Questions to ask a scan-to-BIM provider
| Question | Why it matters |
|---|---|
| How do you check the model against the point cloud? | Look for a documented deviation check, not only a visual review. |
| How do you treat areas the scan did not capture? | You want inferred elements labeled, not drawn as measured. |
| Can you work to our Revit template and standards? | The model should drop into your project without rework. |
| Which accuracy standard do you work to? | A named reference such as USIBD LOA makes the requirement testable. |
| What do you deliver besides the model? | A query list and QA notes are part of a trustworthy handover. |
| How are revisions handled? | Agree how many rounds are included and how they are tracked. |
| How is our building data kept confidential? | Scans of occupied or sensitive buildings need careful handling. |
DigitiseIT has been providing CAD expertise to global customers for over 25 years. Our Revit services and Matterport and point cloud services support architectural teams working on existing buildings. We build the model from your scan data and brief; your design team remains responsible for decisions made from it, and for any site verification.
Have an existing building to model?
Send us a sample of your point cloud or scan data, tell us what the model is for, and we will come back with a scope and quote before any work starts.
Related articles
References
- Building Design + Construction, “Renovation work outpaces new construction for first time in two decades” (November 8, 2022), reporting AIA architecture firm billings data. bdcnetwork.com
- BIMForum, Level of Development (LOD) Specification, 2025 edition, Part I. bimforum.org
- LiDAR News, “Enhancing Laser Scanning with USIBD LOA v3.1 Specifications” (version 3.1, January 2025; original specification 2014). The specification itself is published by the U.S. Institute of Building Documentation. lidarnews.com
