If you are cohabiting and are buying your first home – it is highly recommended that you sign an agreement that sets out the rules. Who pays what, what happens if you split up, etc.
If you are cohabiting and staying in a property that is owned by one of the partners, it is preferable to have a written co-habitation agreement. Who pays what, what happens if you split up, etc.
This is a generic affidavit template.