Ask any site manager what their working tolerance is and you will get an honest answer: whatever the next trade can absorb. Concrete is set out to ±15 mm, steel arrives at ±10 mm, and the façade contractor carries a 25 mm adjustment in the bracket because everyone knows they will need it.
That system works. It has built most of the world. But it only works because every joint is made once, by a person, looking at the specific gap in front of them.
Repetition changes the maths
A production line makes the same joint fourteen times a week. If each unit is 8 mm out in a random direction, a stack of six is out by an amount nobody can predict, and the façade that was drawn to a 20 mm joint arrives to find 4 mm in one place and 41 mm in another.
So we do not work to a tolerance that the next trade can absorb. We work to ±3 mm across the diagonal, checked on a laser jig before the unit is released from the frame station, because the alternative is to design an adjustable connection at every interface — and adjustable connections are exactly the site labour we are trying to remove.
What ±3 mm actually costs
It costs jigs. It costs a setting-out bay that does nothing but measure. It costs about forty minutes per unit in QA time, and it costs the willingness to reject a frame that is 5 mm out rather than pack it and move on.
What it buys is a façade that can be drawn to a 20 mm joint and stay there, a bathroom pod that lands on its drain every time, and a stitching crew that carries torque wrenches instead of grinders.
The number that matters
On our last four residential schemes, the number of inter-module connections that required on-site adjustment was 11 out of 4,320. That is the whole argument for factory tolerance, expressed as a single ratio.