invoice to 6% (house older than 10 year)

Hello,

Example: I have an invoice for a customer whose house is older than 10 years, so to 6%.

Question 1: Do I need to include something on the invoice as proof that the house is older than 10 years?

Question 2: Does the customer only pay 6% on the goods on the invoice or also on the labour costs (the entire invoice)?

Thanks in advance.