Just about anyone can contract to buy a bag of crisps because the consequences and sums of money involved are relatively trivial. We are all more or less capable of doing different things, so the circumstances are relevant to the test of capacity. Just because a client is not really skilled at something doesn't mean that they cannot or should not do it; they might just need help. For example, many householders in Scotland now have a computer but most don't understand and frankly do not care about the hardware and how it actually works. They know that they want the computer to do certain things and perform to a certain standard, be prepared to pay a certain amount and may ask a friend who has an interest in computers to help them buy what they need.
Clients will need capacity to make quite complicated decisions and this would include buying a house, so not everyone will be able to do this, but some may with assistance. Clients would have to be able to understand (broadly) the terms of their mortgage and be able to instruct a solicitor. Clients may have to make decisions about how much to offer for a house, when they want to move in, and who will lend them the money. Some solicitors will be content to simplify the legal terms to enable people to understand. Often solicitors are simply trusted to advise on the bits that need to be known.
In making a contract a client must:
If someone has limited capacity - see Power of Attorney section.
If someone has no capacity - see Guardianship Orders, Intervention Orders, Renting to relatives, Trusts.
Alzheimer Scotland - Action on Dementia - has produced a list of solicitors who have expressed an interest in mental health matters