You can request a home visit on your pip form (or ring for one if you have already been sent an appointment for an assessment centre,) but you will need a letter from your GP which backs up your request for one.
The letter needs to make it clear that you need a home visit because to go to an assessment centre would cause you considerable physical and/or emotional distress and could cause a deterioration in your condition. They still do not have to agree to your request.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems