Invoicing to yourself

You should not charge yourself a profit when invoicing yourself! You should also not charge yourself for installation.
You simply purchase the goods and invoice yourself for the privately used goods at the purchase price with the correct vAT. Do not create a 6% vAT invoice by charging for installation, as this could cause problems.

OK, thanks for the info! So purchases + 21% vAT even though it is a renovation....

You have to be careful with this, because for a number of installations you can get a subsidy from the Flemish government, provided you have an invoice from a certified installer. You will therefore have to weigh up on a case-by-case basis whether it is worth charging yourself 6% vAT for installation (on which you will then be taxed) on the one hand, the discount from the supplier on the other, and the amount of the Flemish subsidy.