Primary residence vs rental allocation

the right answer is whatever method would most properly allocate expenses which really isn't an answer

Typically, you can divide the expenses for rental and personal use based on either:

• The square footage of the rental space versus that of your entire house (e.g., 250 square foot room rented / 2,500 square foot house = 10% of expenses are deductible).
• The number of rooms rented out versus the total number of rooms in your house (e.g., two rooms rented / 10 rooms in the house = 20% of expenses are deductible)

• for the following type of expenses
• Mortgage interest 
• Repairs
• Improvements
• Homeowners insurance
• House cleaning or gardening services(?) the IRS may deem landscaping gardening as 100% personal
• Trash removal
• Snow removal
• Security system costs
• Depreciation

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certain other expenses may be more properly allocated based on the number of people in the home

like utilities

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in your situation with use of the basement that complicates matters. However, I think square feet, including the basement, is most appropriate. 

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others may have a different opinion