from the horses mouth...
1. For the original installation, this will continue to retain its existing tariff rate and eligibility period (section 5.8 of the guidance document).
2. The new extension will be treated as a separate site, except with regards to the tariff band which will be determined using the aggregated capacity of both the extension and the original site. However as noted in point 1, this will not have any impact on the tariff or eligibility period of the original installation.