For the nine months ended September 30, 2020, compared to 2019, our Stabilized proportionate property net operating income increased by $0.5 million, or 1.6%. This increase was attributable primarily to a $0.8 million, or 1.7%, increase in rental and other property revenues due primarily to higher average rent of $50 per apartment home. Additionally, renewal rents increased by 4.7%, whereas new lease rents decreased by 1.1%, resulting in a weighted-average increase of 1.8%. The increase in Stabilized proportionate property net operating income was offset partially by a $0.3 million, or 1.8%, increase in property operating expenses due primarily to an increase in real estate taxes and insurance, offset partially by a $0.3 million decrease in controllable operating expense.
Separate Portfolio proportionate property net operating income for the nine months ended September 30, 2020, compared to 2019, decreased by $0.6 million, or 1.8%, due primarily to a $0.5 million decrease in rental and other property revenues attributable primarily to a 0.8% decrease in average daily occupancy and a $0.3 million increase in bad debt expense, offset partially by a higher rental rates.
Other proportionate property net operating income increased by $4.0 million, or 158.3%, for the nine months ended September 30, 2020, compared to 2019, due to the acquisition of Hamilton on the Bay in August 2020 and 1001 Brickell Bay Drive in July 2019.
Non-Segment Real Estate Operations
Operating income amounts not attributed to our segments include offsite costs associated with property management and casualty losses, which we do not allocate to our segments for purposes of evaluating segment performance.
For the nine months ended September 30, 2020, compared to 2019, non-segment real estate operations were consistent.
Depreciation and Amortization
For the nine months ended September 30, 2020, compared to 2019, depreciation and amortization expense increased by $13.0 million, or 29.0%, due primarily to depreciation and amortization of assets at 1001 Brickell Bay Drive, acquired in July 2019.
Interest Expense
For the nine months ended September 30, 2020, compared to 2019, interest expense, which includes the amortization of debt issuance costs, increased $5.6 million, or 43.0%, due primarily to higher average property-level debt outstanding and the issuance of the note payable to AIR OP in October 2019.
Mezzanine Investment Income, Net
On November 26, 2019, we loaned $275 million to the partnership owning Parkmerced Apartments. For the nine months ended September 30, 2020, we recognized $20.6 million of income in connection with the mezzanine loan, net of transaction cost amortization.
We have accrued all interest amounts due as required by GAAP. Our loan is secured by approximately $300 million of borrower equity. In the event we determine that a portion of the loan or accrued interest is not collectable, we will cease income recognition and, if appropriate, recognize an impairment.
Unrealized Loss on Interest Rate Option
During the nine months ended September 30, 2020, we recorded a $2.1 million unrealized loss on our interest rate option, which we entered into during June 2020.
78