deed of trust? will it affect mortgage application

bunglesabout

Hi all, I was hoping that someone might be able to give me some advice on whether a deed of trust will affect a mortgage application.
Basically I and my partner intend to buy a house with my mother. She will live with us and that has meant that she can help to fund a large enough house for all of us, and also free up some of the money from the sale of her house so that she can enjoy her retirement without having to struggle with continuing mortgage payments of her own.
It also buys me some peace of mind that she won't feel alone in her later years.
I have been with my partner for over 6 years now and she and my mum get on great, so its a situation that I hope will benefit everyone.
We all began the process of selling our respective homes last year and are now living together in a rented property whilst searching for a new home.
Recently we viewed a nice house and put in an offer which was accepted, and now we have applied to a bank for a mortgage.
My mother intends to supply the lion share of our deposit (80,000) whilst we will add 10.000 to this and are hoping to get a mortgage of around 150,000. I was told that due to my mothers age, she would not be able to go on the mortgage with us, but it is important to me that her share of the house is protected and a friend said that you could get a deed of trust written up stating what % of the house she owned. This seemed great to me, but I never really thought about whether it would affect our mortgage application and am now worried that this may be seen in a negative light by the bank and they may say no to our application.
Does anyone know if this will be the case, or what the banks response is likely to be?
How would this affect deed affect the process?
Any advice would be gratefully received.

Bungle