If you have a shed or something that you are using for your home office, you can list it as a separate asset.
You can add the space to your home space and use it as a portion of your entire square footage of "structures" on your home. This will allow you to allocate things like the electric and insurance based on size of your home (if you choose that method) If you have separate utilities, separate insurance and everything else for the structure, than you can just choose to have it as its own standalone structure. Both ways would be considered reasonable methods to use.
Home Office
When you walk through the beginning questions about your home office, you will come to a screen that says "Do any of these apply to your home office" Put a check mark in the box that says this home office is a separate structure.
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