Our existing geocoders don't do a great job (geocoding is a hard problem), and have an especially hard time at guessing containment in situations like you describe.

One solution is to map your house using iD (the default editor when you click the "Edit" button on osm.org) and add the full address, including the city and postcode. With that extra information, the geocoder should be able to find your address. Feel free to e-mail me if you've got more questions.