Thanks for your enquiry.
I confirm that your question/issue falls under UK law.
The safest thing for you to do is to create a Tenancy Agreement. This not only sets out in black and white the terms on which your wife will be able to live in the property, it also allows you to gain possession of the property at the end of the tenancy agreement, or if your wife breaches her obligations under it. Without any such agreement in writing, there may be problems at a later date in you attempting to evict her.
The normal and trusted tenancy agreement in the UK is called an "Assured Shorthold Tenancy" (AST). It is normally for a term of 6 months or 12 months, and you would have to charge her rent- I appreciate you wanted her to live there rent free, but an AST is valid is valid provided the yearly rent is more than £250 (outside London) and more than £1,000 (if the property is in London). Therefore, if the property is outside London, you would need to charge her a monthly rent of at least £20.84!
Once the AST has expired 9ie after the 6 or 12 month period), the tenancy continues on a month to month basis until either party gives the other party notice to end the agreement. If you were wishing her to vacate and she didn't, you would have to issue Court proceedings to evict her, but provided it is an AST, the Court will always grant the required Order. (If there is no tenancy agreement in place and you have a mere oral agreement, it may take you 6-12 months to evict her).
A local Solicitor or Letting Agent will be able to prepare an AST for you, without much trouble.
I hope this assists you and answers your question.
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