As-Built Model

What is an as-built model, and how does it differ from the as-built documentation and survey?

An as-built model is a 3D BIM model of a building in the state it was actually constructed. It is corrected for every deviation from the approved design, and each component carries the data a later owner or engineer needs, such as manufacturer, installation date and material build-up. It is not the same thing as the as-built documentation, which is the formal record of completed works that the authorities expect at completion: drawings, certificates, test reports and deviation notes, mostly 2D. Nor is it the as-built survey, which is a geodetic measurement of where the structures stand and at what heights. The three answer different questions. The documentation shows what was done and how it was certified, the survey fixes where it is, and the model shows what the building is made of and how its parts connect, in a form software can query.

Aspect	As-built model	As-built documentation	As-built survey
Main form	3D BIM model with attached data	Drawings, certificates, test reports (mostly 2D)	Measured coordinates, elevations and plan drawings
Question it answers	What is built, and how do the parts relate?	What was done, and how is it certified?	Where is it, and at what height?
Typical producer	BIM modeller, usually starting from the design model	Contractor and designers, collected and checked by the architect	Surveyor, measuring on site
Later use	Facility management, renovation planning, linking to sensor data	Authority file, warranty and inspection evidence	Cadastral registration and boundary records

Why does an as-built model matter at handover of a new building?

Handover transfers the keys, the documents and the start of the warranty period. Within a few months, though, the owner is likely to need to find a valve, a cable run or a fire-rated penetration behind a finished wall, and paper drawings rarely show where the concealed services actually went. A model that reflects the final state lets the facility manager locate those elements, see their installation records and plan maintenance without opening layers that were already closed. The model complements the paper record rather than replacing it. Whether a model is delivered at completion depends on the contract and the client's information requirements, so write the requirement into the brief from the start.

How is an as-built model built during construction?

The most reliable approach is to keep the design model as the working base and record changes as they happen, instead of reconstructing the building from memory at the end. Agree the process early, ideally in the BIM execution plan, and then follow a simple routine:

1. Agree which changes must be recorded, in what format, and who signs them off.
2. Site teams log deviations in a shared issue register, referenced to the model element they affect.
3. Installers send as-installed data for concealed services before the layers are closed, because photographs taken later rarely show routes clearly.
4. Before handover, the model is compared with the as-built survey and the documentation set, then revision-stamped so the owner knows which state it represents.

Keeping a single shared environment for these exchanges, a common data environment, prevents the model from drifting into several copies, each with different changes.

How do you start an as-built model for renovating an old building?

Renovation is a different starting point. There is often no model at all, and the old drawings may be decades out of date or may show a layout that was altered later without any record. The usual approach is a measured survey, most often scan-to-BIM, in which a laser scan produces a point cloud that is then modelled into walls, slabs, openings and services. The scan shows the true geometry, but it is not the whole story, so the sources are best combined:

Source	What it gives	Main limitation
Old drawings and permits	Original intent, layout and structural concept	May not match what was built or altered later
Terrestrial laser scan	Measured surfaces and true geometry	Sees only visible surfaces, so hidden layers need checking
Photogrammetry	Textured images for quick capture	Accuracy depends on control points and lighting
Opened sections and site checks	Wall build-up, structure and concealed routes	Local only, and it damages finishes if overused
Geodetic survey	Position against boundaries and levels	Too coarse for interior components

A raw point cloud has no meaning on its own. It is a set of points, not walls or ducts, so someone has to classify them, and areas hidden behind furniture, in shafts or during works in progress will be missing. Plan for those gaps rather than hiding them.

How reliable does an as-built model need to be, and how do you state it?

Reliability is what the Level of Development framework describes: how far an element may be relied upon, which is different from how detailed it looks. An element measured on site and verified should carry a higher level than one copied from an old drawing. The safest practice is to tag every element with its source, so that a designer can see at a glance whether a dimension was measured, inherited from the design model or assumed. An assumption presented as a measurement is the most common way an as-built model misleads the people who use it later.

What mistakes make an as-built model less useful than it looks?

Several failure patterns recur, and most are cheap to prevent if they are named early:

• Modelling the design, not the building. The approved model is copied and the site changes are never entered.
• Unlabelled scan gaps. Areas the scanner could not reach are filled with guessed geometry without a status flag.
• No owner after handover. Nobody is assigned to update the model, so it is accurate for one day and out of date after the first repair.
• Unanchored coordinates. The model is not tied to the survey coordinate system, so it cannot be overlaid on the site or on the cadastral plan.
• Excess detail. Every fixing is modelled, which slows the file and adds nothing a facility manager can query.

The fix for most of these is the same: decide the purpose of the model before modelling starts, then record only the detail that serves it. For a renovation, the purpose is usually to replace assumed dimensions with measured ones. For a new building, it is usually to hand over a record that matches the building the owner actually received.