To place a double garage on a semi that has a width of 6.8m of land is just not feasible and will be an eye sore on the aesthetics of the street, with the town planning looking to open up fronts of houses as opposed to making them more closed in.
Losing another car spot off the street is also a problem, given the limited spaces. The house at present has 2 car spots, so not sure why for a semi this would be possible or in the interest of further development in the street.