As-Built Drawings: What They Are and How Accurate They Get
As-built drawings document how a building was actually constructed, not how it was designed: walls, gridlines, floor levels and MEP systems in the position they really occupy after construction. They are produced by measuring the building itself, and the most reliable way to measure it today is 3D laser scanning, which captures the geometry as a point cloud, validates it and turns it into architectural, structural and MEP drawings.
For our as-built drawing service we publish millimeter-level accuracy of ±2 to 4 mm (about 5/64 to 5/32 in); the overall range of our terrestrial scanners is ±2 to 10 mm depending on the equipment and site conditions (Scan to BIM). A small project can be delivered in about a week; a complex industrial facility usually takes three to five weeks, with the exact schedule in the proposal.
8-minute read. You will come away knowing what an as-built drawing really is, how one is produced, what accuracy you can require in writing, which files you receive and what to ask for so the scope doesn’t fall short.
Almost every architecture firm has a file labeled “as-built” that nobody measured. It is the design set with the superintendent’s red-line markups from closeout, cleaned up in CAD. It works as a reference, but the moment someone designs a renovation or an addition on top of it, the differences nobody marked start to show up.
The pattern is easy to recognize. The architect taking on a 1980s building finds the walls are not where the drawings put them. The project manager discovers a duct that shifted sixteen inches and was never drawn. The structural engineer asks for the actual elevation of a mezzanine slab and gets “per drawings,” which is not the same as “per field conditions.” In all three cases the drawings were clean. The building simply didn’t match them.
At Foundtech we capture built reality with 3D laser scanning: more than 200 projects across Europe and the Americas, more than 10 million m² (over 100 million sq ft) modeled, work under ISO 19650, and an alliance with the Swiss firm BIM Facility AG. On the projects we deliver, starting from measured geometry instead of outdated drawings translates into up to 35% less rework and 25% time savings, based on the results we report. If you want to see what each type of finished drawing looks like, we cover that in as-built drawing examples; this article stays with the question that comes first: what as-builts are and how to get them right.
What as-built drawings are and how they differ from design drawings
An as-built drawing represents what exists, obtained by measuring the building, while the design set (as-designed) represents what was meant to be built. The gap between the two can hold shifted gridlines, different finished floor elevations, walls out of plumb and MEP runs that followed the path the structure allowed rather than the one the construction documents showed.
That is why a red-lined set falls short: it only includes the changes someone saw and chose to mark. At the Werdhölzli wastewater treatment plant in Switzerland, the owner wanted to modernize and found there were no up-to-date drawings or engineering documents; with a facility that complex, a traditional survey would have taken too long. The whole plant was scanned, and the models built from that point cloud became the basis for the architects’ and engineers’ design work.
The cost of working without that data doesn’t stay with the design team. The NIST GCR 04-867 study put the cost of information that fails to move cleanly between design, construction and operations in the U.S. capital facilities industry at $15.8 billion a year, with two-thirds of it absorbed by owners and operators during operations and maintenance. An inaccurate as-built is exactly that kind of lost information, and someone pays for it after handover.
How as-built drawings are produced today
There are four common ways to arrive at an as-built, and they don’t support the same decisions. Before choosing one, it helps to know what each records and what it misses.
| Method | What it records | What it misses | When it’s enough |
|---|---|---|---|
| Red-lined design set | The changes someone marked in the field | Everything nobody reported | Recent construction with no significant changes, as a reference |
| Tape, laser distance meter and field sketches | Spot dimensions, element by element | Plumb, square and level over long runs; error accumulates | A small, accessible space or tenant suite |
| Construction progress photos | That something exists and what it looks like | Distances: a photo has no coordinates | Visual backup for a change that was already measured |
| 3D laser scanning | Millions of points with coordinates for every visible surface | Anything hidden behind ceilings, walls or floors | Buildings with a history of changes, multiple disciplines, or an investment that depends on the geometry |
The last row is the one we work with, but the table doesn’t say the others are wrong: for a small, recent space, a tape measure is still the cheapest option and good enough. What scanning changes is that the measurement is stored in a point cloud anyone can revisit months later without going back to site.
Our as-built drawing service follows four stages, always in the same order. First, the on-site 3D scan, which usually doesn’t require shutting down operations because the equipment captures from a distance. Second, geometric and technical validation of the captured data. Third, producing the drawings from that point cloud. Fourth, digital delivery, ready for maintenance or renovation work.
What accuracy to really expect
The accuracy we publish for as-built drawings is ±2 to 4 mm, and our terrestrial scanners range from ±2 to 10 mm depending on equipment and conditions. That number needs a careful reading: it is how precisely the building is measured, not the tolerance it is drawn to.
Between the measurement and the drawing there are decisions. A wall out of plumb can be drawn straight or with its deviation; a pipe can be modeled along its exact path or as a reference line. What settles those choices is the agreed level of detail, which BIM models express as LOD (Level of Development), standardized in the BIMForum LOD Specification. We work from LOD 200 (massing) up to LOD 500 (detailed as-built), depending on what each project needs. Requiring LOD 500 across an entire building when you only need certainty on the facade you are about to touch means paying for accuracy nobody will use.
There are physical limits too. A scanner records visible surfaces: it can’t see through a closed ceiling or a wall. Exposed MEP systems are captured in the scan; concealed ones are integrated from earlier drawings or with ground-penetrating radar, which we offer as an add-on service. And it measures geometry, not condition: it doesn’t detect moisture, corrosion or structural damage, and it doesn’t replace an engineer’s judgment about what that geometry means for the project.
What you receive from an as-built survey
An as-built survey delivers architectural, MEP and structural drawings, and can include evacuation plans with fire extinguisher locations. The standard format is DWG (AutoCAD) plus PDF, so the files open in the software your firm already uses and you are never locked into working with us.
If the project needs more than drawings, we build the model from the same point cloud in Revit (RVT) or Archicad (PLA) through our BIM modeling service, along with free viewers such as BIMx or Autodesk Viewer so anyone on your team or the owner’s side can review it without a license. If you are still deciding between drawings and a model, 3D vs. BIM software explains which tool fits each kind of model.
How long an as-built survey takes
A small project can be delivered in about a week and a complex industrial facility usually takes three to five weeks, with the exact schedule included in the proposal (Scan to BIM). Capture is the fast part: a laser scanner covers thousands of square meters a day. The time goes into the office work, where the point cloud is validated and drafted.
On our service page we publish that drafting from the point cloud is up to 50% faster than with traditional methods. What stretches a project is rarely the scanner. It is a poorly defined scope, like realizing halfway through drafting that the concealed MEP on a floor nobody asked for was needed too.
A real project, step by step
Früh Verpackungstechnik AG makes flexible packaging for medical and pharmaceutical products in Switzerland and needed to expand its production space without reliable information about its facilities or time for a traditional construction project. The existing facilities were scanned, the point cloud was modeled in detail in Revit, 2D drawings were generated from the model, and the result became a digital twin carrying the building’s as-built characteristics, which served as the basis for planning the expansion (Früh Verpackungstechnik AG case).
For a sense of scale on a survey like this, at Migros Millas we documented roughly 11,000 m² (about 118,000 sq ft) by combining interior scans and drone captures, in facilities that had no drawings or MEP documentation, with the goal of improving maintenance and the design of the facilities.
What to do on Monday before you request an as-built
Define what you need it for. An as-built for an interior renovation is not the same as one for MEP coordination or for feeding a BIM model. The use determines which drawings you request and at what level of detail. If yours is a renovation, as-built plans for architectural renovations explains what to survey before you start.
Pick the area, not the building. Surveying every corner at maximum detail is the most expensive and slowest route. Start where not knowing costs the most: the facade you are about to work on, the level where the addition goes, or the spot where drawings and field conditions already clashed.
Gather what you already have. Earlier drawings, even outdated ones, help integrate concealed MEP and keep you from paying twice for the same information.
Get the scope in writing before the price. Which drawings, in what format, at what level of detail, on what timeline and how concealed elements are handled. Two proposals with different scopes can’t be compared on price.
As-built drawings: frequently asked questions
Who should deliver as-built drawings, the contractor or a third party?
Typically the contractor delivers them at closeout. If the building is older or changes went undocumented, an independent survey that measures what exists today is the safer choice.
Can I use a scan that was done a few years ago?
As a starting point, yes, but any construction after the scan isn’t in the point cloud, so those areas need to be validated again.
Do I need Revit to use the deliverables?
No. The standard is DWG and PDF, and if you request the model, it can be reviewed in free viewers without a BIM software license.
A simple way to tell whether this is your problem: if someone at your last OAC meeting said “we need to field-verify that” and the item is still open in the minutes, your project is running on a measurement nobody has taken yet.
Tell us which area of your project you need to confirm before you work on it, and we’ll review the scope, timeline and deliverables of the survey with you. Request a quote for your as-built survey.
