1. Put safety first
Keep away from unstable areas. If there is immediate danger, injury, fire, flooding, a gas smell or risk of collapse, contact emergency services or the relevant utility.
2. Record what happened
Take wide and close-up photographs and video. Note when the problem was first seen, what work was taking place and who witnessed it.
3. Report it immediately
Notify the building owner and site manager in writing. If a particular operation may be causing further harm, ask for that operation to pause while the position is assessed.
4. Prevent further loss
Take reasonable temporary steps to protect people, property and belongings. Avoid permanent repair or redecoration before inspection unless necessary for safety.
5. Allow prompt inspection
Give reasonable access for the building owner and relevant advisers to inspect the alleged damage.
6. Decide how repairs will be arranged
You may allow the building owner to arrange remedial work or prefer your own contractor. If arranging the work yourself, comparable quotations can help establish a reasonable figure.
Safety first. Evidence second. Cause and responsibility can be considered once the immediate position is under control.