Hi, I'm in the middle of building a new work office space (home office, detached addittion) and need to learn how to deduct those expenses

[Irene2805]

Once the structure is complete you can deduct it as a home office expense.  You cannot deduct the expenses of building the structure.

To enter the information on a home office please follow these steps:

1. Click Income and Expenses and then click the Start/Revisit box in the Self-Employment section.
2. On Your 2020 self-employed work summary screen, click on Edit next to your business. 
3. On the Here's your [business] info screen, click on the box Add expenses for this work.  
4. On the Tell us about any expenses screen, scroll down to the Less Common Expenses section and mark the  radio button next to Home Office.  Click Continue at the bottom of the screen. 
5. On the screen asking if you have a home office, mark Yes and click Continue.
6. Continue through the screens, entering the requested information about your home office.
7. When finished you will see the Home office summary. 

[Mike9241]

once complete and put into use you can depreciate the cost of the office space. the cost can not include what you consider the value of your labor if you're doing part or all of the construction yourself. since this is business property the depreciable life is 39 years not the 27.5 for residential real estate.