CAD versus Revit for Existing Building Surveys

An architect receives an old floor plan, a handful of elevations and a deadline for concept design. The immediate question is rarely whether CAD versus Revit is better in the abstract. It is whether the available information represents the building well enough to support the next decision.

For existing buildings, the answer depends on the intended workflow, the level of coordination required and the reliability of the source survey. CAD drawings and Revit models solve different problems. Choosing the right output at the outset helps avoid duplicated modelling, uncertain geometry and costly corrections once design work is under way.

CAD versus Revit: the practical difference

CAD is a broad term, but in building documentation it usually refers to 2D drawing outputs such as floor plans, elevations, sections and roof plans, commonly delivered as DWG files. Each drawing is created in a defined view of the building. It is clear, familiar and efficient for communicating measured information.

Revit is a Building Information Modelling platform. Rather than drawing each view independently, the modeller builds a coordinated digital representation using elements such as walls, floors, roofs, doors and windows. Plans, sections and elevations are generated from that model. Amend a wall in the model and related views update with it.

That distinction matters. CAD is generally the more direct way to document what has been measured. Revit is designed to create a connected model that can support design development, coordination and information management. Neither is automatically more accurate. Accuracy comes from disciplined site capture, careful interpretation and quality control before the output is issued.

When CAD is the right existing-condition output

A well-structured CAD survey remains the most practical choice for many projects. It gives the design team dependable base information without imposing a particular modelling approach. For feasibility studies, planning drawings, measured surveys, lease plans, internal layout work and straightforward refurbishment, 2D CAD is often exactly what is needed.

CAD is also effective when the team needs a fast, clearly defined record of existing geometry. A drawing can show the relevant dimensions, levels, features and annotations without requiring the recipient to manage a model, its worksets, families or parameters. It can be referenced into design software and used as an underlay for new proposals.

For listed buildings and irregular properties, CAD can offer another advantage: it does not force historic construction into standardised object categories. A bowed wall, uneven roof line or decorative opening can be represented honestly in a drawing, with the appropriate notes and conventions, rather than being simplified to fit a generic library component.

This does not make CAD a lesser deliverable. It is often the most proportionate output where the objective is accurate record information rather than a fully coordinated BIM workflow. The key is that layers, linework, units, drawing origin and file structure are organised for immediate use by the project team.

When a Revit model earns its place

A Revit model becomes valuable when the existing building needs to be understood as an interconnected three-dimensional system. This is common on complex renovations, major extensions, coordination-heavy commercial work and projects where several disciplines need to work from a shared digital reference.

The benefit is not simply that the building appears in 3D. A properly constructed existing-conditions model allows the team to cut sections where needed, test proposals against the fabric, coordinate services zones and maintain consistency across drawings. It can reduce the time spent rebuilding the same geometry in separate packages.

Revit is particularly useful where levels, vertical relationships and hidden spatial constraints influence the design. Split-level buildings, complex roof forms, atria, historic structures with irregular geometry and buildings with repeated but non-identical components can all benefit from a model-based starting point. In these cases, a reliable model gives designers a clearer basis for decisions that would be difficult to make from isolated plans alone.

However, a Revit model should be commissioned to a defined level of development and purpose. An LOD100 massing model is not suitable for setting out detailed interventions. An LOD300 existing-conditions model may be appropriate for coordinated design, while LOD400 requirements call for a more exacting scope and should only be requested where the project genuinely needs that level of detail.

More model detail is not always better. It requires more interpretation, modelling time and client review. The right question is: what decisions must this model support?

The real issue: source data and scope

The most polished Revit file cannot compensate for an incomplete survey. Equally, a precise point cloud does not become a usable design tool until it has been interpreted into the drawings or model the team needs.

For dependable existing-condition documentation, the capture method should reflect the building’s complexity. High-accuracy 3D laser scanning is especially valuable where there are irregular walls, non-orthogonal spaces, heritage features, difficult roof geometry or limited access for repeat visits. The point cloud provides a measurable record from which CAD drawings or Revit geometry can be produced and checked.

The project brief should establish what is included before site work begins. That means agreeing the required plans, elevations, sections and roof information; deciding whether ceiling heights, floor levels and service-visible elements are needed; identifying inaccessible areas; and clarifying the expected Revit LOD where BIM is required.

Accuracy should also be discussed in practical terms. Tolerances, survey control, units and the treatment of visible versus concealed construction all affect how a file can be used. Existing buildings are rarely perfectly square, plumb or level. Good documentation records meaningful variation rather than quietly correcting it.

Avoid treating CAD and Revit as competing choices

Many successful workflows use both. A project may begin with a measured CAD package for rapid feasibility work, then develop into a Revit model once the design route and coordination requirements are confirmed. Alternatively, a Revit existing-conditions model may be delivered alongside extracted 2D plans, elevations and sections for teams or consultants who work primarily in CAD.

The right arrangement depends on who will use the information and when. If the lead architect is working in Revit, a carefully scoped model can remove a substantial amount of internal set-up work. If the immediate requirement is a planning application based on measured existing drawings, CAD may be more efficient and easier to brief.

It is also worth separating the model from the visualisation. A Revit model is not automatically a survey-grade record simply because it looks convincing in 3D. Nor does a detailed point cloud eliminate the need for modelling judgement. Objects need to be represented at a level appropriate to the project, and unusual building fabric may require bespoke modelling rather than assumptions based on standard components.

Questions to settle before commissioning documentation

Before requesting a quote, define the design decisions the documentation must support. Is the immediate need planning, feasibility, detailed design, consultant coordination, heritage consent or construction information? Will the team work in CAD, Revit or both? Which areas are most geometrically sensitive, and what information would be expensive to discover late?

It is equally useful to identify the delivery standard. A CAD file needs agreed drawing scales, layer conventions and required views. A Revit model needs a specified version, LOD, coordinates, modelling assumptions and the extent of any parameters or classifications. These details turn a generic request for a survey into a useful production brief.

For complex, listed or irregular buildings, early discussion with a specialist documentation partner can prevent scope gaps. Space Captures uses measured capture and structured modelling to provide design-ready CAD and Revit outputs that reflect the building as it stands, not as an old drawing suggests it should stand.

The best choice is the one that gives your team dependable geometry without paying for information the project will not use. Start with the decisions ahead, then commission the drawings or model that let those decisions be made with confidence.