If ten lines of data and a browser can carry the monitoring loop, what exactly are you buying when you procure a digital twin? A closing argument for owners, with the clauses I would write.
Willy Tai · Smart Cities · AI · Sustainability · September 2026
This series began with a picture and ended up somewhere less comfortable. Part 1 showed a governed operating loop running in a browser. Part 2 deleted most of the model and the loop kept working. Part 3 rebuilt the building's geometry from one photograph and a table of observed and assumed values. Put those together and a question falls out that I have never seen written into a tender: when an owner buys a digital twin, which part of the money buys the twin, and which part buys polygons?
The people who build the most detailed models are arriving at the same conclusion. At a SimBuild 2026 session on the PAE Living Building in Portland, the team described being unable to reach their own building's data after handover, and taking nine months to reverse-engineer what they had designed. Their answer was to write data delivery into the contract, in an open, machine-readable form, or watch the intelligence evaporate at practical completion. That is the conclusion the Gemini Principles reached in 2018 from the policy side: a twin is judged on purpose, trust and function. Fidelity costs money, and the specification should say which decision that money serves.
Part 2 reduced the job to three answers: what is broken, what state everything is in, and roughly where it sits. Everything the Estate Twin does to catch a fault, route it through rules, put a model's proposal in front of a named person and log the outcome runs on relationships and telemetry. The stepped wireframe of the Lattice view is enough geometry for all of it. That is the test I would put in front of any twin proposal: which of the three answers does each expensive component improve, and by how much?
In my experience, most twin tenders specify a platform, a level of detail for the model, an integration count, and a dashboard. They rarely specify who owns the point schedule, whether the relationships between assets and the spaces they serve exist as data anyone else can read, or what happens to the time series when the contract ends. The vendor is rewarded for polygons and integrations, both of which are countable at handover, and nobody is rewarded for the relationship layer, which is the part that keeps paying for the next fifteen years. The vendor built what the tender asked for, and blaming them for it misreads the incentive.
Illustrative drafting only. Adapt these with your own lawyer and quantity surveyor before they go near a real contract.
As at August 2026, Green Mark In-Operation certificates run for three years, and since April 2025 the certificate names the stage at which the assessment was made. Buildings caught by the Mandatory Energy Improvement regime must cut EUI by ten percent within three years of submitting their improvement plan, and hold it for a year. Check current BCA guidance before relying on either. Both make the twin's data something the owner will have to show a regulator. The evidence the regulator will ask for comes out of the relationship layer. Nothing in the geometry produces it.
The Estate Twin on this site was built to make the argument pressable, and each part has said what the demo can and cannot show. The twin is small, its data is synthetic, and its geometry came from a photograph that does not exist. None of that weakens the clauses, because they rest on the loop. The loop is the part I have delivered at estate scale.