There's definitely some good advice here, but it might be helpful to note that we as demisexuals typically need friendship to have a relationship. That perspective can blind us to how many allosexuals are, and I've known many who only place people in one of two categories, friend or relationship. For these type of people, if you express an interest, and they aren't interested, you will likely lose that friend.

That said, I definitely think having a partner who is also a best friend is the only thing that would make sense for us.

In regards to your situation, if the person is allosexual, you might want to ask them how they view friendships that become more, without hinting it's you. Finding how they view things can help you decide if you want to take that gamble.