In the interest of administrative simplification, the obligation to use a filled in 6% VAT certificate will cease to apply as of 1/7/2022. Since 01/01/2022, it is sufficient to include a standard statement on your invoice that places the responsibility for applying the 6% VAT rate on the client.
There are two steps you'll need to complete to put this statement on your invoice and apply the 6% VAT rate.
Open the invoice template you want to change via 'Settings' > 'Corporate style'. In the column with keys on the right hand side, choose the category 'order' and the key "$Order.sixPercentVatCertificate$". Click anywhere in your template to add the key there. The following text will be added:
VAT rate: In the absence of a written dispute within a period of one month from receipt of the invoice, the customer shall be deemed to acknowledge that (1) the works are performed on a dwelling of which the first occupation took place in a calendar year that is at least ten years prior to the date of the first invoice relating to those works, (2) the dwelling, after the performance of those works, is used exclusively or primarily as a private dwelling, and (3) the works are supplied and invoiced to a final consumer. If at least one of these conditions is not met, the normal VAT rate of 21% shall apply and the customer shall be liable for the payment of the tax, interest and fines due in respect of these conditions.
Now switch on the option via your settings. In the menu on the left hand side, go to 'Settings' > 'General' > 'Advanced'. Under 'Adding extra text to an invoice with a VAT rate of 6 percent' you turn on the setting. If you haven't added the key to your template, you will get an error. You can still add the key to your template via the keys on the right hand side.
The text will be displayed on all invoices with a 6 % VAT rate. If there are invoices where this doesn't apply you can temporarily disable this option when creating an invoice. You can turn the switch back on when you've finished.