I question you claiming a home office for that portable building. If it's portable, then it's not taxed by your local taxing authority that assesses property taxes. So the portable building itself is a business asset that is listed as such in the Business assets section. It's not a home office, because that portable building is not taxed by your local property tax authority. Therefore it's listed as a business asset and is depreciated over 40 years just like any other major business asset. You will not be claiming a home office per-se, because you are not using any space as a home office, that you pay property taxes on.
Your cost basis for this asset will be your total purchase price of the portable building plus the cost of what you put into it to insulate it, wire it up with electricity, water, gas or whatever other utilities that were installed in the building after you placed it on your property.
For the utilities utilized in the building you can figure that based on what your utility costs were before the building was hooked up. Basically, total up your 2016 utility costs and get that figure. Lets say for example, it cost you $3000 in 2016 for water and electricity in your house for a year. That averages out to $250 a month.
Now add up your utility cost for the months after you installed the building. Lets say you got things fully operational Jun 1st of 2017. So add up your utilitiy costs from Jun through Dec of 2017. Lets say that total for those 7 months comes to $2,100. Over 7 months that averages out to $300 a month. That's a 20% increase on average, per month.
So for 2017 your business expense for utilities would be $50 a month, or $350 for those last 7 months of 2017. Then for 2018, figuring that your utility cost will not be exactly the same of course, you can figure 20% of whatever you spend in your household for utilities, to be for the business.
You already know how to add the business as a business asset that will be depreciated over 40 years. So here's how to set this up in TurboTax for the utilities side of this equation.
Work through the business profile section and select the option to indicate "My business has a different address" and continue. For the street address just add an "A" to the end of it (since it's your home address) and that will be fine. For example, if your address is 123 AnyStreet Drive, make the address for the business 123A Anystreet Drive. If par chance any mail is actually addressed to 123A Anystreet drive, the post office will still deliver it to 123 Anystreet Drive. So that's really not an issue.
Now, when you get to the Business Expenses section, in the sub-section for "Other Common Business Expenses" there's a category for utilities. That's where you'll enter that percentage of your utility costs that will apply to the business.
