Do I put rent/mortgage deductions for the amount of space I use exclusively for my business in the business tax return or my personal tax return? Which area does it get entered?
Enter the Office in Home expenses ONLY in the business expenses section and the personal use portion of the common expenses will carry to the Sch A automatically ... do NOT ever enter an amount twice ... read the screen instructions carefully.
If you have an eligible Home Office the expenses can be deducted based upon the square footage of the office as a percentage of the square footage for the home. Property taxes, mortgage interest, utilities and assets to be depreciated including the home office itself can be used.
Review this IRS website for Home Office eligibility - https://www.irs.gov/businesses/small-businesses-self-employed/home-office-deduction
IRS Publication 587 Business Use of Your Home - https://www.irs.gov/pub/irs-pdf/p587.pdf
TurboTax support FAQ for a home office - https://ttlc.intuit.com/questions/1901224-can-i-take-the-home-office-deduction
The details are on the screen. You are asked for two types of expenses. Whole House Expenses are what you paid for the whole year. Generally you will have values for those expenses that are more than $0.
"Home Office Only" expenses are those expenses you paid for the *OFFICE* *ONLY*. While not rare, it is very uncommon to have anything greater than zero when using a part of your primary residence for business.