Collect the certificate of compliance on completion or professional opinions, as-built drawings, the BER certificate, the safety file, commissioning and test certificates, warranties, and the wastewater or connection agreements.
The handover pack is what a future purchaser's solicitor will ask for. Assembling it at completion, while everyone is still contactable, is far easier than reconstructing it years later.
Store both digital and hard copies, and keep the folder with the property records rather than with personal papers.