If you are a W-2 employee, you must meet three tests to take the home office deduction. You must work regularly from home (have no other main place of work); you must work exclusively from your home office, meaning you set aside a part of your home for the office and don't also use it for personal use; and you must work at home for the convenience (or requirement) of your employer, not just your own convenience.

Then, your home office deduction is a percentage of all expenses that are attributed to the whole house, on a square foot basis including gas and electric, insurance, repairs, mortgage interest, property taxes, and depreciation (wear and tear). 

However, flat rate services like home telephone and internet may not be deductible at all.  There are two arguments: One argument is that they are never deductible because they are flat rate services and your work use does not actually increase your cost, so your cost is not attributable to work and is not deductible. The other school of thought is that you can deduct the percentage of work use, but only if you have detailed records that can prove the amount of work use -- details of call minutes and internet megabytes that you can assign to business or work purposes.  So they are not deductible for most practical purposes.

Your home office expenses are transferred to form 2016 with any other unreimbursed work expenses, and then they are itemized deductions subject to the 2% rule, so your actual deduction will be reduced based on your other tax situations.

Note that when you sell the house, you must pay back (recapture) any deduction you took or could have taken for depreciation as a capital gains, even if the sale otherwise qualifies for the capital gains exclusion.  You may want to consider the simplified home office deduction method which does not involve you in depreciation recapture. 

https://www.irs.gov/businesses/small-businesses-self-employed/simplified-option-for-home-office-dedu...