An excellent point. In order to accomplish this it would be necessary for ownership of the house to be a tenancy in common rather than a joint tenancy.
Property ownership for a couple is very often a joint tenancy. This means that in essence both partners own the whole of the property; it grants them certain rights, for example, both must agree to a sale but most importantly, if one dies then ownership of the property is automatically conferred to the survivor. This happens no matter what any Will says, it is automatic. It confers protection to a surviving spouse by ensuring they cannot be rendered homeless but it can be problematic if there is a desire to leave part of the property to another beneficiary.
Under a tenanacy in common, each part-owner owns their share independently, and may do with it as they wish. They may sell it or leave their share to whomever they wish in the Will.
The most common useage of this is in strategies to reduce liability to Inheritance Tax.
However, it can also help protect property against forcible sale to pay for care fees. For example, if your dad were to leave his share to you, it is removed from your mum's assets. Further, it is very hard to sell part of a house and at the very least would reduce the market value of your mum's share when assessed as an asset.
There are drawbacks to tenancies in common, so it's impportant to get legal advice on it.
The property would be exempted as an asset if, say, your mum had to go into care but your dad remained living in it. This is an automatic protection for spouses/civil partners of any age, also any other family member but only if they are aged 60 or over. The problem would come if your dad also had to go into care, or passed away, at that point the property would no longer be protected