The increase in operating expenses was primarily driven by a 20.8%, or $0.8 million, increase in repair and maintenance expense, partially offset by a 10.6%, or $0.7 million, decrease in personnel expense as a result of fewer employees, and a 9.3%, or $0.9 million, increase in real estate taxes, which was primarily due to higher assessed valuations.
The operating margin (property net operating income divided by property rental income) was 73.6% and 73.9% for the three months ended September 30, 2019 and 2018, respectively.
Nine Months Ended September 30, 2019 vs. Nine Months Ended September 30, 2018
NOI for our Same-Store Community properties increased 3.6%, or $7.8 million, for the nine months ended September 30, 2019 compared to the same period in 2018. The increase in property NOI was primarily attributable to a 3.7%, or $10.7 million, increase in property rental income, which was partially offset by a 3.9 %, or $2.9 million, increase in operating expenses. The increase in property income was primarily driven by a 2.8%, or $7.6 million, increase in rental rates and an 11.7%, or $3.6 million, increase in reimbursement and ancillary and fee income. Physical occupancy remained the same at 96.8% and total monthly income per occupied home increased 3.7% to $2,210.
The increase in operating expenses was primarily driven by a 20.2%, or $2.0 million, increase in repair and maintenance expense, partially offset by a 7.9%, or $1.4 million, decrease in personnel expense as a result of fewer employees, and a 6.5%, or $1.8 million, increase in real estate taxes, which was primarily due to higher assessed valuations.
The operating margin (property net operating income divided by property rental income) was 74.2% and 74.3% for the nine months ended September 30, 2019 and 2018, respectively.
Non-Mature Communities/Other
The Operating Partnership’s Non-Mature Communities/Other represent those communities that do not meet the criteria to be included in Same-Store Communities, which include communities recently developed or acquired, redevelopment properties, sold or held for disposition properties and the non-apartment components of mixed use properties.
Three Months Ended September 30, 2019 vs. Three Months Ended September 30, 2018
The remaining 5.2% or $4.2 million, of our total NOI during the three months ended September 30, 2019 was generated from our Non-Mature Communities/Other. NOI from Non-Mature Communities/Other decreased 26.5%, or $1.5 million, for the three months ended September 30, 2019, compared to the same period in 2018. The decrease was primarily attributable to a $0.9 million decrease in NOI from redevelopment communities and a decrease of $0.5 million in NOI from non-residential/other.
Nine Months Ended September 30, 2019 vs. Nine Months Ended September 30, 2018
The remaining 7.1%, or $17.1 million, of our total NOI during the nine months ended September 30, 2019 was generated from our Non-Mature Communities/Other. NOI from Non-Mature Communities/Other decreased 19.2%, or $4.1 million, for the nine months ended September 30, 2019, compared to the same period in 2018. The decrease was primarily attributable to a $1.9 million decrease in NOI from non-residential/other, a $1.6 million decrease in NOI from redevelopment communities and a decrease of $0.9 million in NOI from sold communities.
Gain/(Loss) on Sale of Real Estate Owned
During the nine months ended September 30, 2019, the Operating Partnership did not recognize any gains on the sale of real estate. During the nine months ended September 30, 2018, the Operating Partnership recognized a gain of $70.3 million on the sale of an operating community in Orange County, California.
Interest Expense
During the three and nine months ended September 30, 2019, interest expense increased by $2.4 million and $7.0 million, respectively, as compared to the same periods in 2018, primarily due to the conversion in 2018 of the Advances (to)/from the General Partner capital balance into an unsecured note payable and an unsecured revolving note payable, both with the General Partner.