Possible to have a Deed of Trust showing different shares of the mortgage principle?

name_user6

Hi,

My partner and I are due to remortgage and will be doing some big renovations around the same time. We're tenants in common with 50:50 mortgage payments and 50:50 equity, covered by a Deed of Trust. We want to keep the same equity split for the foreseeable future (perhaps but maybe not changing on marrying/having kids).

She has more wealth than I do but I will earn a materially higher salary in the coming years. I would have to nearly clear my ISA of several year's allowance for the building work and personally would prefer drawing a larger mortgage to fund the renovations. She has more reason to pay down the mortgage and reduce her borrowing. 

I understand the Deed of Trust can lay out different mortgage payments, but can it also dictate different shares of the mortgage principle owed? So could it be constructed not only so that I pay more each month, but also so that I repay more of the principle in other events such as selling up.

I appreciate this needs lawyer input, but wanted to know if there's any precedent for this.
From my research it looks like you can technically get mortgages independently as tenants in common, but I can't find lenders who will actually do that. Plus I might want to cover some of her mortgage payments e.g. during maternity leave which would be easier in a joint mortgage.

name_user6

*years'(!) name_user6 said:

clear my ISA of several year's allowance