20%, to $95.2 million for the nine months ended September 30, 2019, from $79.5 million for the nine months ended September 30, 2018, primarily due to the acquisitions described above. Total operating expenses increased by $1.8 million, $4.0 million, $2.1 million, $2.2 million, $3.4 million, $1.6 million, $0.8 million and $0.3 million, respectively, from the acquisitions of Pima Center, Circle Point, The Quad, Greenwood Blvd, Camelback Square, Canyon Park, Cascade Station and 7601 Tech properties. Park Tower operating expenses also increased by $0.6 million due to the higher occupancy at that property. Washington Group Plaza operating expenses decreased by $0.8 million due to its sale in March 2018 and Plaza 25 operating expenses decreased by $2.1 million due to its sale in February 2019. Sorrento Mesa decreased by $1.5 million due to the sale of the 10455 Pacific Center building of the Sorrento Mesa property in May 2019. General and Administrative Expenses increased by approximately $2.4 million, of which $1.1 million was the result ofone-time expenses and accruals incurred as a result of the assignment fee income earned during the nine months ended September 30, 2019 and the balance related to higher payroll costs. The remaining operating expenses were modestly higher in comparison to the prior-year period primarily due to higher occupancy at the properties.
Property Operating Expenses. Property operating expenses are comprised mainly of building common area and maintenance expenses, insurance, property taxes, property management fees, as well as certain expenses that are not recoverable from tenants, the majority of which are related to costs necessary to maintain the appearance and marketability of vacant space. In the normal course of business, property expenses fluctuate and are impacted by various factors including, but not limited to, occupancy levels, weather, utility costs, repairs, maintenance andre-leasing costs. Property operating expenses increased $6.2 million, or 17%, to $42.8 million for the nine months ended September 30, 2019 from $36.6 million for the nine months ended September 30, 2018. The increase in property operating expenses was primarily due to the acquisitions described above. The acquisition of the Pima Center, Circle Point, The Quad, Greenwood Blvd, Camelback Square, Canyon Park, Cascade Station and 7601 Tech contributed an additional $0.7 million, $2.0 million, $1.0 million, $1.2 million, $1.1 million, $0.4 million, $0.3 million and $0.1 million, respectively, in additional property operating expenses. Park Tower operating expenses also increased by $0.2 million due to the higher occupancy at that property. Washington Group Plaza decreased by $0.8 million due to the sale of that property in March 2018 and Plaza 25 decreased by $1.1 million due to the sale of that property in February 2019. The remaining property operating expenses aggregate to an overall $1.1 million increase in comparison to the prior-year period.
General and Administrative. General and administrative expenses are comprised of public company reporting costs and the compensation of our management team and board of directors as well asnon-cash stock-based compensation expenses. General and administrative expenses increased $2.6 million, or 46%, to $8.4 million for the nine months ended September 30, 2019 compared to $5.8 million for the nine months ended September 30, 2018. Of this increase, $1.1 million can be attributed to theone-time expenses and accruals incurred as a result of the assignment fee income earned during the nine months ended September 30, 2019 as described above and the balance of the increase was primarily attributable to higher payroll costs.
Depreciation and Amortization. Depreciation and amortization increased $7.1 million, or 19%, to $44.1 million for the nine months ended September 30, 2019 compared to $37.0 million for the nine months ended September 30, 2018, primarily due to the addition of the Pima Center, Circle Point, The Quad, Greenwood Blvd, Camelback Square, Canyon Park, Cascade Station and 7601 Tech properties. These increases were partially offset by a decrease at Washington Group Plaza, Plaza 25, and the 10455 Pacific Center building of the Sorrento Mesa property due to the sale of those properties.
Other Expense (Income)
Interest Expense. Interest expense increased $5.5 million, or 32%, to $23.0 million for the nine months ended September 30, 2019, compared to $17.5 million for the nine months ended September 30, 2018. The increase was primarily due to interest expense related to acquisitions. Interest expense for the Circle Point, The Quad, Greenwood Blvd, Canyon Park and Cascade Station property level debt increased by $1.2 million, $0.9 million, $0.8 million, $1.0 million and $0.3 million, respectively, and the interest on the line of credit increased by $2.3 million as a result of acquisitions funded by our $250 million Unsecured Credit Facility. These increases were partially offset by a $0.2 million and $0.5 million, respective decrease in the Washington Group Plaza and Plaza 25 debt as a result of the sale of those properties and the extinguishment of its property level debt.
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