many good points are already mentioned above.
I'm guessing first that the property you're actually talking about is not
inside Washington, DC.
if it were a single lot in the District that big, then the cost $$ of an electrician might not be an issue for anyone fortunate enough to own such a property!
given the distances involved, and without the benefit of knowing or seeing your site, it's harder to choose among the two alternatives you've offered.
what is the size & age of the existing service to the house? how big is your existing main panel? does it have the physical space and electrical capacity to accommodate a new 30 / 40 / 50 / xx amp circuit?
you might also want to take some voltage readings to be sure that your existing service can handle that new and additional load.
how is the existing house served from the road? is it buried, or aerial with poles?
does that service route come anywhere closer to your parking site?
if the parking site is "along the way" you might
be able to branch a second service entrance onto it.
again, that would depend on the type / size / capacity / utilization of the existing facility.
if the parking site is not "along the way", then a new main service to the AS site might be practical, too.
would that new main service be aerial, or buried?
are there any building / zoning requirements that demand buried main service when it's "new" construction? will your building department call it "new" or "an addition'?
for installing new buried service, from both the street or the house, what are your terrain and soil conditions like? a clean straight grassy straight line in sandy soil, or will it be rocks, trees, and hard clay??
who is your power provider? for aerial service, they will sometimes
provide at no charge or reduced cost xx number of poles and xx feet of aerial feed, and you would be responsible for the rest. for buried service, usually
you will be responsible from their pole on the street to your structure or service location. how much additional $$ cost each month for having that second meter?
there's another thread here that may be useful to you - http://www.airforums.com/forums/f447...ice-65688.html
and some suggestion for a small book you might wish to read here - http://www.airforums.com/forums/f447...tml#post861265
and, if I may ask, how were you planning water supply and waste services if this is going to be "permanent extra housing"??
I know that I've probably given more questions than answers, because all these elements might need to be considered before you can create an informed and educated solution to your need.
in any case, do it according to Code, good practice, and with real engineering, and permits and inspections. this project will be too expensive to use "rules of thumb" and other sop engineering.