Keep the important parts of the job together.
A good record is useful during the project, at handover and if the two sides later remember something differently.
Building projects generate information quickly. The useful goal is not to save every conversation forever. It is to keep the information that explains what was agreed, what changed, what was paid for and how the project finished.
Keep the starting documents.
Save the accepted quote, scope, relevant specifications and the main agreement. These establish the baseline against which later changes can be understood.
Keep decisions that change the job.
If a message or conversation changes the work, materials, price, timing or responsibility, record the decision clearly. Variations should be identifiable rather than buried inside a long message thread.
Use photographs with purpose.
Progress photos can record work before it is covered up, unforeseen conditions, agreed changes and snagging items. Dates and a short description make them much more useful later.
They help the job run more clearly day to day. Their value as evidence is a second benefit, not the only reason to keep them.
Worth keeping together:
Keep the record balanced.
A project history should not exist solely for one side to build a case against the other. A shared record is more useful when both client and trade can see the important agreed information and the sequence of decisions.
This is the thinking behind the Trust In Trade Project Hub.
The aim is one project record rather than important information scattered across email, messaging apps, paper documents and memory. If support is needed later, that same record can help establish the facts more efficiently.