I sold a rental property in 2017. I refinanced in 2007, but I never included to cost basis in 2007 taxes. Can I add this to this year's cost basis or is it too late?

[MichaelDC]

What part of the refinance do you want to add to the basis? Points? Closing costs?

[arleyl80]

Closing costs. I understand points are deducted over the life of the loan.

[MichaelDC]

Much in the same way as points, closing cost are added to the basis are depreciated. When you sell, the law states that basis is reduced by depreciation allowed or allowable. If you don't take depreciation, the IRS will reduce your basis when sold anyway as if you had taken depreciation. So even if you don't take depreciation the IRS treats it as if you did; the IRS will recapture depreciated allowed are allowable at a rate of 25%.
The easiest thing to do would be to enter a new asset placed in service the date of refi 2007 and depreciate over 27.5 years (residential). TurboTax will deduct prior depreciation (allowable) and add the rest to the basis for the sale.
The next step, if you want, doesn't seem worth for a small deduction is to go back and amend the returns you can (2014-2016).
The last resort is filing Form 3115. TaxGuyBill is the expert here. https://accountants-community.intuit.com/questions/1238149