You certainly could register as a Childminder working from an alternative premises in the past, though it could have changed. First point of contact would probably be your local EYDCP/CIS. You can find their contact details using the Childcare Link website.
If it is still allowed, then you will need to meet the requirements, you can't for example only be caring for the children who live at those alternative premises, you also have to be willing to take children from other families.
If you work for just one family, in that families home, then IR will consider that you are a Nanny... and are therefore Employed, rather than self-employed.
Some publications that could help you. Note: these are for those living in England. If you live in Wales/Scotland/Norther Ireland etc then things will be a little different.
Childminding - Is it for you?
Guide to Childminder Registration
National Standards: Childminding
National Standards Guidance
National Standards Guidance - Revisions
With regards to space, the best way to work out how much space you have is to make several 1m x 1m squares from newspaper. Lay these out over the room(s) you would be using for childminding and you will get a good idea of how many square metres you have.