What you are referring to is a capital allowance scheme in designated areas i.e. BMW region.
Section 50 merely creates a huge loss in the first year of renting i.e. your rent for the year less the qualifying expendiure on the accomadation. Rental losses can only be used to shelter rental income both current year and carried forward
hence the reason why a s.50 or a s.23 investment property is only usefull to a individual with more than one property