If the subject is perfectly still (and since it's a building let's hope that's true... otherwise you should run for cover) AND if the camera is perfectly still (e.g. on a "solid" tripod -- and I do emphasize "solid") THEN you can leave the shutter open on your camera as long as necessary to get the shot.
A flash wouldn't carry far enough to be effective on a subject such as a building (you'd need some incredible lighting -- so I wouldn't bother.)
I emphasized "solid" tripod because I have a few tripods. Some are solid, but heavy (wouldn't want to have to go on a long hike and have to carry them) and some are quite light. My lightest tripod flexes quite a bit -- it's "springy". That means I have to use a remote release (or use the self-timer mode on the camera) to trigger the shutter and if it's windy, just the wind will keep the tripod from settling. Basically if you need a long exposure you want to make sure the tripod doesn't have a vibration or you'll get a blurry shot.
Once you've got a solid mount, go ahead and experiment. While you could use a low f-stop, that'll provide a narrow depth of field. You may just want to use a middle or high f-stop because now that the camera is solid, time is no longer an issue. Keep the ISO low to prevent "noise" in the image.