Straight lines can be a problem. A sharp, ruled edge looks artificial when placed among freehand lines, particularly in a loose sketch. Try placing the ruler or other straight-edge parallel to the line you want, so that you can run the side of the finger holding the pencil, not the pencil tip itself, along the edge to guide your line. This can take a bit of practice to get right, but it's a useful way to create a more natural-looking straight line. Alternatively, try drawing from the elbow and shoulder rather than the wrist, moving the arm straight up and down - if you are pivoting the hand from the wrist, you get a curve. Drawing straight lines freehand takes a bit of practice - a deft, confident line wins over a tentative one.
Of course, I'm thinking here of a cottage in a landscape or a simple urban wall and window. A cityscape in three-point perspective is going to be a little more challenging. A mathematically correct projection of an architectural plan - mm, nope, I did arts, not engineering! Our guide to CAD might be able to help you there, beginning with articles such as this one on Drafting Simple Lines.
More on Drawing Architecture
Draw a Brick Wall in Pen and Wash
A rough sketch of the same wall
Brick and Stone Textures
A Sketch of an Old House
Rough Pencil Sketch of Dover Castle
Great Architectural Drawings