If, as with the original poster, a part of a house is purchased by notarial deed, the purchase contract along with options (e.g. upgrading) as well as the BB become part of the notarial contract. This means that if the original poster upgrades their equipment and this is included in the notarial contract, the real estate transfer tax also increases. In the present case, the real estate transfer tax is therefore applied to the house + land because the original poster receives everything from one source.
However, I have read here in the forum and on the NET that special requests, even if added later or not included in the notary agreement, can be subsequently taxed by the tax office with real estate transfer tax. Is that correct? When is a construction service not subject to VAT?