What does builder need to know to provide a quote for converting office building to flats?

To make sure you get a full proper quote and don't incur any 'extras' as the job goes on, they will need copies of the plans as approved by planning permission and also the building regulation details (electrical layout plans are good too). If you've had a survey done this is also handy so any major defects can be considered and of course a site visit will need to be done. Louise UK Property Services

You need to give the builder as much information as possible. Detailed drawings/approved plans, building regulations and survey. You will need to have planning permission for change of use which should detail any conditions the planners may require. Most builders could only give you a ball park estimate without these details and a full spec of the end finish IE fittings, bathroom, kitchen spec will need to be agreed.