Sequencing could be tested during tender. Request a simple programme showing investigation and approvals first, then demolition, concealed services, waterproofing, testing, finishes and final commissioning. For specified materials, ask for lead times and acceptable alternatives before work...
I would initially reserve around 15%, then reduce it only after opening-up risks and contractor exclusions are clearer. Ask whether the price includes replacing concealed plumbing, repairing substrates after tiles come off, distribution-board or wiring work, moisture remediation, waste removal...
The next step is to narrow the sample rather than enlarge it indiscriminately. Use comparable warehouse locations and condition bands, then compare recent completions with active and withdrawn stock. If the +9.5% survives that split, it is more persuasive. If it disappears, the apparent rise was...