Hey James, done plenty of Basic Service(s) without plans on homes that are new-ish.
I am sure others will chime in - I think the point is the 6 month rule just defines whether a home is new or existing. Basic Service technical procedures apply to both.
A Basic Service applies to all eligible homes and defines procedures requiring onsite measurements.
The Construction Upgrade Service preludes construction and is intended for design stage planning and therefore requires plans as per technical procedures 1.4.4.
Of course if a Construction Upgrade Service is conducted then not much measuring is done at the "Basic Service" part of providing a rating (if a builder so chooses to take that route).