Get UK Law Questions Answered by Verified Experts
The postition is that if you were to move in and contributed "rent" directly to him (and not paid to his mortgage company) then provided the amount you pay is not more than you would otherwise pay in rent privately elsewhere and you do not make other capital expenditure (eg significant sums to improve the property and it's value) then you would not be able to make any kind of claim on the property.
Nevertheless, domestic arrangements as to finance are inevitably fluid so if they shoe were on the other foot I would be advising your fiancee to ask you to execute a cohabitation agreement for the additional security that it provides.
Once you have been married for considerable time though the house would be included in the matrimonial asset pool over which you would negotiate settlement, but so would your other house so it would presumably even out to an extent.
If this is useful please kindly click accept so that I may be rewarded for my time. You will be free to ask follow up questions. Kind regards,