There are specific IRS tests that must be met in order to deduct a home office:
1. Your home office must be used on a regular and exclusive basis as your principal place of business or
2. Your home office must be where you meet or deal with customer, clients or patients or
3. If a separate structure, the structure must be used exclusively and regularly in connection with your business

If you are an employee, your home office must meet a "convenience of the employer" test, meaning your employer has an office for you but would rather you use your home office for employer business reasons.

Principal Place of Business
Principal place of business test is not failed if you have more than one trade or business using that same office;
Principal place of business test is not failed if you have more than one business location for your trade or business;
Principal place of business test is not failed if you use your home office exclusively for management or administrative activities as long as these activities are substantial;
Principal place of business generally applies to sole proprietors;
Principal place of business test is failed if you use the home office occasionally;

Storage of Inventory
In order to obtain a home office deduction for the storage of inventory used in a trade or business, your home office storage facility must be your only fixed location and
The space must be separately identifiable and
The space must be used regularly.