Does a separation agreement need to be registered on title to protect a spouse's interest in the home?

Not strictly, as between the spouses themselves — a properly signed separation agreement is generally binding on the spouses who agreed to it regardless of whether anything is registered against title. Registration becomes relevant mainly for a different reason: giving notice to third parties, like a future buyer or lender, of an agreed arrangement affecting the property, so those third parties can't later claim they were unaware of it.

Registering a separation agreement, or a notice referencing it, against title can help prevent a spouse from dealing with the property in a way that's inconsistent with what was agreed, since a search of title would reveal the arrangement to anyone considering buying or lending against the home. Without registration, a spouse relying only on the private agreement between them could face a more difficult situation if the other spouse tried to sell or refinance in a way that conflicts with it, particularly if a third party had no notice of the agreement. Because whether registration is worthwhile depends on the specific terms of the agreement and the level of trust between the spouses, anyone finalizing a separation agreement involving the home should discuss with their lawyer whether registering something against title makes sense in their situation.