What the phrase is doing
Sacred space is simply space you have marked as different from ordinary space, for as long as you need it. The marking can be physical (clearing a surface, lighting something, putting a cloth down) or entirely internal (deciding, and meaning it).
Permanent, temporary, portable
Some Wiccans keep a permanent altar. Many make space temporarily, then clear it away again, which is the usual reality of shared housing. Others work portably, with a small tin or pouch. A permanent altar is an option, not a requirement.
Practising in a shared house
Privacy helps, but a practice that only works in an empty house tends not to happen. Headphones, a closed door, an early morning, or simply doing something quietly while others watch television are all legitimate solutions.
Cleaning is not cleansing, but it helps
Tidying a surface before you use it is not a magical act in itself, and it is still the step that makes the difference most often.