Being slightly pedantic, the first thing to say is that what used to be called applications for 'ancillary relief' are, of course, now applications for 'a financial remedy'.
Joining, or to use the new terminology (again) 'adding' parties to proceedings for a financial remedy is not uncommon, as often there are other parties who have an interest in the outcome of the proceedings, usually because they have, or claim to have, an interest in the property involved. Typical third parties may be mortgagees, shareholders in a family company, or simply other family members.
Adding parties to proceedings for a financial remedy is governed by rule 9.26B of the Family Procedure Rules 2010, which came into effect on the 6th April last. As rule 9.26B(1) states:
(1) The court may direct that a person or body be added as a party to proceedings for a financial remedy if–The court may add a party either on its own initiative or on the application of an existing party or a person or body who wishes to become a party.
(a) it is desirable to add the new party so that the court can resolve all the matters in dispute in the proceedings; or
(b) there is an issue involving the new party and an existing party which is connected to the matters in dispute in the proceedings, and it is desirable to add the new party so that the court can resolve that issue.
An application must be made in accordance with the Part 18 procedure on Form D11, supported by evidence setting out the proposed new party's interest in or connection with the proceedings.
If the court makes a direction for the addition of a party, it may give consequential directions about (a) the service of a copy of the application form or other relevant documents on the new party; and (b) the management of the proceedings.