It takes roughly 20 minutes to get into a focused mental state where you can truly concentrate on your writing. This means that whenever you are interrupted from your writing, even if it's only for a few seconds, that interruption will cost you a minimum of 20 minutes of productive work time. That's crucial wasted time that you could have used to create new ideas and new content.
Therefore, it is critical that your spouse, kids, roommate, or whoever you live with understands and respects the true costs of interrupting you. Even seemingly minor interruptions can kill your productivity by costing you time far out of proportion to the length of the interruption itself.
To be successful at writing, you must make sure the people around you respect your time and your attention.