When is a Declaration of Trust enough on its own?

Hi Sorry I've read through the forums and some responses state that a declaration of trust is sufficient on its own and some suggest they must be accompanied by a Form 17 - could you just clarify for me please in particular if a property is owned through a joint mortgage ? And secondly if the example was a wholly owned property by one individual and income is 100% declared under their spouse. thank you

Hi,
If a property is jointly owned by spouses, then a 50/50 split is assumed.  
To declare unequal beneficial interests a Form 17 and declaration or deed of trust will be required by HMRC.
Please see TSEM9852 - Form 17 rule - how couple make declaration for more information. 
If a property is owned by one individual they can assign beneficial interest to their spouse using a Deed of Assignment or Deed of Trust.
Most types of Trusts giving an interest in a property to someone who is not the legal owner now need to be registered with HMRC via the Trust Registration Service.
Guidance can be found at Register a trust as a trustee.   
Thank you.