My house is a long way off. Here 'off' is a/an -

Off can be used in the following ways:
as an adverb:
- He waved and drove off.
- She took her coat off and hung it up.
- My house is a long way off.

as a preposition:
- She got off the bus at the next stop.
- Keep off the grass.

as an adjective:
This meat is definitely off – I can’t eat it.I’m having an off day.

Source: macmillandictionary.com