What Is a BIM Model and Do I Need One?

What Is BIM?

BIM stands for Building Information Modelling. It's a way of representing a building as a three-dimensional digital model — not just a set of 2D drawings, but a fully structured three-dimensional object where every element (walls, floors, windows, doors, structure, services) is represented as an intelligent component with properties, dimensions and relationships to the elements around it.

Unlike a 3D model in SketchUp or a visualisation tool, a BIM model is a working design document. Floor plans, elevations, sections, schedules and 3D views are all generated directly from the model — they're different representations of the same underlying data, not separate drawings that need to be kept in sync manually.

What Software Is Used?

The dominant BIM platform in the UK is Autodesk Revit — used by the vast majority of architectural practices, structural engineers and M&E consultants working on BIM projects. At SurveyX, all our as-built BIM models are produced in Revit 2025.

Revit models can also be exported to IFC format (Industry Foundation Classes) — an open standard that allows the model to be opened and used in other BIM applications including ArchiCAD, Bentley AECOsim and Trimble products. We provide IFC export as standard alongside the native Revit file.

What Is an As-Built BIM Model?

An as-built BIM model is a BIM model of an existing building — representing the building as it currently is, not as it was originally designed or as it will be after refurbishment. It's the starting point for design work on existing buildings, providing the design team with an accurate three-dimensional reference of the existing conditions.

As-built models are produced from measured survey data — either from traditional measured survey measurements or, more accurately, from 3D laser scan data. The laser scan point cloud is loaded into Revit and used as the reference dataset for building the model, ensuring every wall, floor, ceiling and opening is positioned correctly in three-dimensional space.

What Does an As-Built BIM Model Contain?

The content of a BIM model depends on the agreed Level of Development (LOD) — a standard that defines how much detail is included. For as-built survey models we typically work to:

• LOD 200 — elements modelled as generic shapes with approximate dimensions. Walls, floors, ceilings, roofs and openings are modelled to the correct approximate dimensions. Suitable for early-stage design and spatial planning. Quicker to produce and lower cost.
• LOD 300 — elements modelled with accurate dimensions, positions and orientations taken directly from the survey data. Wall thicknesses, ceiling heights, structural elements and opening sizes are all modelled to match the as-surveyed building. This is the standard most architects require for planning applications and detailed design. Our most common delivery standard.

At both levels, the model includes:

• External and internal walls with correct thicknesses and positions
• Floors and ceilings at correct levels
• Roof geometry including pitches, ridges and eaves
• Doors and windows at correct positions and sizes
• Stairs and structural elements where visible
• 2D drawings extracted from the model — floor plans, elevations and sections

When Do You Need an As-Built BIM Model?

You need an as-built BIM model when:

• Your architect works in Revit and needs the existing building as a Revit model to design over — rather than importing 2D DWG drawings as flat underlays
• The project involves BIM coordination — the structural engineer and M&E consultant are also working in BIM and the teams need a federated model environment
• The building is complex and the design team needs to work in three dimensions from the start rather than building up a 3D model from 2D drawings
• You need accurate schedules from the model — door schedules, room area schedules, window schedules extracted directly from the model data
• The client or contract requires BIM — increasingly common on commercial refurbishment and public sector projects

When Are 2D CAD Drawings Sufficient?

Not every project needs a BIM model. For many planning applications, residential extensions and smaller refurbishments, a well-structured set of 2D DWG drawings is entirely sufficient. The architect can work from the DWG files directly without the overhead of managing a Revit model.

A BIM model adds cost and production time compared to 2D drawings. If your project doesn't require the three-dimensional coordination or schedule capabilities that BIM provides, a measured survey with 2D CAD output is usually the more practical choice.

If you're not sure which is right for your project, the key question is: is your design team working in Revit? If yes, an as-built Revit model will save them significant time. If they're working in AutoCAD or a 2D environment, a DWG survey is what you need.

How Is an As-Built BIM Model Produced?

Our scan-to-BIM workflow covers everything from site to delivery:

1. The building is scanned using the Leica BLK360 to capture a complete, millimetre-accurate point cloud
2. The point cloud is registered and processed in Autodesk ReCap
3. The RCP file is linked into Revit as a three-dimensional reference dataset
4. All building elements are modelled to the agreed LOD using the point cloud as the reference
5. 2D drawings are extracted from the model and set up on drawing sheets
6. The Revit file, IFC export, point cloud and PDF drawings are delivered via Dropbox