Most writers I know plan to do their writing, at least most of it, at home. So I guess if you consider your writing activity a business, the temptation is to deduct every household expense you can conceive of as a business expense. After all, you work at home.
But you need to be careful and be aware of the rules. Remember, tax law distinguishes between what is personal and what is business. Generally, rent, mortgage payments, repairs on a residence and the like are not deductible. They are considered personal living expenses. You have to have some place to live. In tax administration such expenses are usually personal.
In order to get a business deduction for the use of your home you must use part of it (1) exclusively and regularly as your principal place of business, as a place to meet or deal with patients, clients or customers in the normal course of your business, or in connection with your trade or business where there is a separate structure not attached to the home; or (2) on a regular basis for certain storage use such as inventory or product samples, as rental property, or as a home daycare facility.
Now, the assumption is that you're working as a proprietor, not as an employee of someone else. If you are an employee who receives a W-2, in order for you to claim a deduction relative to the use of your home you must be able to prove, in addition to the other things mentioned above and talked about below, that the use of your home is for the convenience of your employer and that the portion of the home so used is not rented by the employer.
Now let's talk about all of that. First, exclusivity. Note that. It means entirely. It means you don't use it for anything else. Supposedly, you don't eat there, cook there, or do your wash there. You write there. Now, I know there's going to be someone out there who is a freelance writer who specializes in writing articles about cooking and producing cookbooks or some such thing. And they are saying, what about me? My writing about cooking and producing cookbooks entails more than just putting words down on a piece of paper or in a computer. Okay. I can buy that. But it's more likely that you use whatever area you use for cooking as an adjunct to your writing also for personal purposes. Where do you cook your meals that you consume? Or that your family consumes? The more you look for exceptions to what "exclusive" means in this way, the more lack of clarity it has as to "exclusivity" and the more chance you have of having it questioned.
Regular use means that you use the area for your writing on a regular basis, not incidentally or occasionally. Regular versus incidental and occasional. That's the test; only regular use qualifies.