Surcharge for controlled residential ventilation in the consulting contract okay?

Are you a private person? Then the VAT is already included in the offer from September 1. Otherwise, he would have made you a [Schwarzarbeitangebot] and he certainly does not want to discuss that in detail with his tax office.

He is not allowed to make you an offer without VAT at all. Therefore, if it is not stated separately, it is included.

Kind regards,
Andreas

No, she isn't. He actually first gave me a "quote" (strictly speaking not a quote for me, but a cost breakdown) for the BAFA about the total costs (because the funding depends on that). Later, he gave me a supplementary offer for a subsidized heating system + controlled residential ventilation vs. standard service, meaning what I have to pay additionally to the house price. Since these didn't match, I complained and it turned out that he forgot to include the VAT in the BAFA breakdown, as he thought BAFA only subsidizes the net amount.