I'm not sure which Magnolia you are referring to (there are dozens of species and varieties deciduous and evergreen). But to get back to your question, I don't think it's a good idea to plant ANY tree too close to your house or other trees (too much competition for water--magnolias are water-loving). Magnolia roots tend to be thick without too many fibrous roots. It is also true that they tend to be shallow rooted. They are also not the best trees to recuperate from transplanting (so you pretty much want to avoid transplanting wherever you do put it). I made a mistake of planting a M. grandiflora too close to my house. It eventually was removed as it was growing into the house (branches not the roots). When you plant, you really have to consider what the tree will look like in 10 to 20 years (assuming it thrives and continues to grow). If you reply with specifics (the species you like, your zone, sun exposure, etc.), someone may be able to give you a safe minimum planting distance. Good luck!