I fear B's contribution is a little off.
Since X is contemplating the transfer (presumably by way of gift) of some beneficial interest in the investment property to her husband, and assuming they are living together as a couple, then by default following that transfer (in any proportion) the income therefrom will be assessed on them 50:50 - this in accordance with ITA 2007 s 836. No notification would be required in order to achieve this.
- for HMRC's position on this - it's not perfect but it works OK for this issue.
This default approach will NOT apply if they ARE in partnership (as distinct from merely holding the property jointly - and you need to be careful with adopting a partnership approach as SDLT (and, by implication, Scottish LBTT) can get quite tricky with Property Investment Partnerships) or if they ARE letting the property jointly and the property qualifies as "Furnished Holiday Accommodation" - I am assuming X would have mentioned this already if it DID so qualify.
X and Mr. X may however CHOOSE to displace the default treatment and be assessed on their actual respective shares in the property by use of Form 17. Evidence of that actual ownership split would be required as a matter of course.
- which goes into a LOT more detail on joint assets, partnerships, etc., and the Form 17 process.
Income distribution normally follows beneficial ownership only when the co-owners are NOT spouses or civil partners. And it may readily be changed.
To summarise, X, if you transfer 10%, 15%, 60%, 90% to Mr. X then it will in most cases (except partnerships, furnished holiday accommodation, etc., as above) be taxed as to 50:50 UNLESS you choose the Form 17 route.
Please do not forget the potential pitfall re: SDLT/LBTT and gifts of interests in property subject to mortgage - there's no "spouse exemption" in that regard.