There are two issues you raise which need to be addressed.
The first is the formation of a partnership to enable more of the property income to be allocated to the wife.
Although, as you say, a partnership means the assumed 50/50 income entitlement of s836 ITA 2007 does not apply, this is not the end of the matter. A non-commercial allocation of profits within a partnership is still within the settlement legislation of s624 ITTOIA 2005 and can apply to all partnerships and LLPs. This is a view confirmed by HMRC at TSEM4215.
The allocation of more of the profits to the wife without a corresponding increase in her share of partnership equity simply to avoid income tax would be caught by the settlement legislation. The exception in s626 ITTOIA 2005 for transfers between spouse/civil partners would not apply as this would be “wholly or substantially a right to income” without a corresponding transfer of partnership equity. If the couple’s particular rental properties require personal involvement of time and effort and such work is only carried out by the wife, or most of it is carried out by the wife, then it may be possible to commercially justify a greater share of the partnership profits to be allocated to the wife. How much would depend on the time spent and the nature of the work undertaken.
I would add that whether the particular rental properties constitute a “business” that meets the requirements of the Partnership Act 1890 depends on the facts involved. Although not an issue actively pursued by HMRC at the moment, there is a published view from HMRC at PM131800 which states:
The letting of jointly owned property does not normally constitute a partnership. Most cases will fall short of the degree of business organisation needed to constitute a business. The provision of significant additional services in return for payment may be an indicator of such business organisation.
The second issue relates to the unrelieved finance costs.
The rules to determine the entitlement to relief of non-deductible costs of a dwelling-related loan for an individual are contained within ITTOIA 2005 S274A and S274AA. Section 274A(3) tells us the relievable amount for a tax year is the total of the individual’s current-year restricted finance costs (if any) for that year in respect of that business and the unrelieved individual’s brought-forward restricted finance costs (if any) for that year in respect of that business.
The legislation restricts the tax reduction for finance costs so that a reduction can only be made against the income tax liability on the same property business to which the finance costs relate.
Although the properties and the individuals carrying on the letting activities will remain the same after the partnership is formed, for income tax purposes the partnership property business is a separate property business to that previously carried on by the individuals (ITTOIA 2005 S859(2)). This rule pre-dates the finance costs restriction and is covered by HMRC at PIM1030.
Therefore, unrelieved dwelling-related loan costs accumulated by the wife will be lost on the formation of the general partnership.
Whilst the wife’s only source of income is property income, she may continue to accumulate unrelieved finance costs. Your clients may want to consider allocating more of the underlying ownership of the property to the wife or transferring other income producing assets to utilise her personal allowance to maximise finance cost tax reductions.
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