Property rental revenue increased by approximately $7.4 million, or 6.4%, to $122.8 million in 2021 from $115.5 million in 2020. The increase was primarily due to (i) a $4.6 million increase related to the deferral of rent and the write-off of deferred rent receivables for tenants that were placed on the cash basis of accounting in 2020 and a decrease in uncollectable operating lease receivables attributable to COVID-19 in 2021, (ii) a $4.2 million increase related to 4747 Bethesda Avenue, West Half, The Wren, 900 W Street and 901 W Street as these properties placed additional space into service, (iii) a $2.7 million increase related to 1770 Crystal Drive, which was placed into service in the fourth quarter of 2020, and (iv) a $1.5 million increase related to the commencement of leases with Amazon at 2100 Crystal Drive and 2200 Crystal Drive. The increase in property rental revenue was partially offset by a $3.4 million decrease related to the Universal Buildings and RTC-West due to lower occupancy and a $1.7 million decrease related to RiverHouse Apartments and The Bartlett due to increased rent concessions and lower market rents.
Third-party real estate services revenue, including reimbursements, decreased by approximately $422,000, or 1.6%, to $26.7 million in 2021 from $27.2 million in 2020. The decrease was primarily due to a $2.0 million decrease in reimbursements revenue related to tenant services projects, partially offset by a $1.3 million increase in development fee revenue primarily related to the timing of development projects.
Depreciation and amortization expense increased by approximately $4.1 million, or 7.7%, to $56.7 million in 2021 from $52.6 million in 2020. The increase was primarily due to a $2.2 million increase related to 4747 Bethesda Avenue, West Half, The Wren, 900 W Street and 901 W Street as these properties placed additional space into service, a $2.0 million increase related to 2345 Crystal Drive due to an increase in tenant improvements and an $801,000 increase due to 1770 Crystal Drive being placed into service. The increase in depreciation and amortization expense was partially offset by a $1.1 million decrease at 7200 Wisconsin Avenue due to the disposal of a tenant improvement in 2020.
Property operating expense increased by approximately $1.2 million, or 3.6%, to $35.0 million in 2021 from $33.8 million in 2020. The increase was primarily due to a $1.6 million increase related to 2451 Crystal Drive for costs incurred for construction management services provided to tenants and a $1.1 million increase related to 4747 Bethesda Avenue, West Half, The Wren and 900 W Street as these properties placed additional space into service. The increase in property operating expense was partially offset by a $674,000 decrease related to 1901 South Bell Street due to costs incurred in 2020 for construction management services provided to tenants and a $567,000 decrease related to the Crystal City Marriott as the property incurred higher costs due to COVID-19 in 2020.
Real estate tax expense increased by approximately $689,000, or 3.9%, to $18.6 million in 2021 from $17.9 million in 2020. The increase was primarily due to a $641,000 increase at 4747 Bethesda Avenue, The Wren, 900 W Street and 901 W Street as these properties placed additional space into service.
General and administrative expense: corporate and other increased by approximately $679,000, or 5.1%, to $13.9 million in 2021 from $13.2 million in 2020. The increase was primarily due to increases in employee compensation and consulting costs, partially offset by declines in share-based compensation expense and temporary staffing costs.
General and administrative expense: third-party real estate services decreased by approximately $3.7 million, or 12.6%, to $25.6 million in 2021 from $29.2 million in 2020. The decrease was primarily due to a decrease in reimbursable expenses related to tenant services projects.
General and administrative expense: share-based compensation related to Formation Transaction and special equity awards decreased by approximately $4.4 million, or 49.9%, to $4.4 million in 2021 from $8.9 million in 2020. The decrease was primarily due to the graded vesting of certain awards issued in prior years, which resulted in lower expense as portions of the awards vested.
Transaction and other costs of $2.3 million in 2021 includes $1.6 million of expenses related to completed, potential and pursued transactions, $439,000 of demolition costs related to 2000 South Bell Street and 2001 South Bell Street, and $222,000 of integration and severance costs. Transaction and other costs of $1.4 million in 2020 consist primarily of integration and severance costs.