Generally speaking, there are fewer legal rights for cohabiting couples than married or civil partnered couples in the event of a relationship breakdown – for example, it can be harder to claim financial support or relief from your ex-partner or a greater share of your 'joint' possessions.
Yet you're on more solid ground when it comes to property you jointly own as a cohabiting couple, be that as joint tenants or tenants in common. Your interest is not diminished by the fact you're not married. However, you may want to consider tenants in common and a deed of trust if your interest in the property is more complicated than a simple 50/50 split (which joint tenancy assumes).
Watch out if you're a cohabiting couple but the property is solely owned by your partner, as you'll have far fewer rights to it in the event of a relationship breakdown than if you were married. You can try claiming a beneficial interest in the property – for instance, if you've been contributing to the mortgage – but this can be tougher to achieve than if you were married.
This is a very complex area of law and there are no hard and fast rules, so if you need support it's best to seek legal advice.