Quality assurance process
It is easy to make mistakes with any modelling software, so having a robust QA procedure within your organisation is key. Ideally there will be someone who can look over your PHPP on a regular basis. Clear mark-ups of treated floor area, heat loss areas, windows and shading make for easy checking, as does keeping previous versions of your PHPP.
If your experience is limited employ a Certifier early on in a project to check what you’re doing, agree any points of contention and provide any hand-holding you may need. Whilst certification usually starts pre-construction, having some experienced input (especially before planning) will ensure you’re on the right track.