What kind of magnolia do you have? They generally don't cause root damage but some are worse then others. Nevertheless, it's planted too close to the house. Magnolias don't take kindly to pruning so I'd keep it to the bare minimum. I don't know what else to tell you. If you try to transplant a magnolia that size, you may loose it under the best of circumstance.

If its a grandiflora the roots will be well under the house by now. They are unlikely to cause any structural damage. The danger would come if there were ever to be a leak in any plumbing pipes under the house as the roots would find them and enter the crack and block the pipe, much like willows would.