Note 8 - Property, Plant and Equipment

As of December 31, 2016 and 2015, our property, plant and equipment, net, consisted of the following (in thousands):

Construction in progress is comprised primarily of tooling and equipment related to the manufacturing of our vehicles, a portion of Gigafactory 1 construction, and related capitalized interest. In addition, construction in progress also included certain build-to-suit lease arrangement for the Buffalo manufacturing facilities acquired through our SolarCity acquisition during the fourth quarter of 2016.  Completed assets are transferred to their respective asset class and depreciation begins when the asset is ready for its intended use. Interest expense on outstanding debt is capitalized during the period of significant capital asset construction. Capitalized interest on construction in progress is included in property, plant and equipment, net, and is amortized over the life of the related assets. During the years ended December 31, 2016 and 2015, we capitalized $46.7 million and $41.5 million of interest expense, respectively.

We are sometimes involved in construction at our leased facilities primarily related to retail stores, service centers, and certain manufacturing facilities. In accordance with ASC 840, Leases, for build-to-suit lease arrangements where we are involved in the construction of structural improvements prior to the commencement of the lease or take some level of construction risk, we are considered the owner of the assets and land during the construction period. Accordingly, upon commencement of our construction activities, we record a construction in progress asset and a corresponding financing liability. Once the construction is completed, if the lease meets certain “sale-leaseback” criteria, we will remove the asset and related financial obligation from the balance sheet and treat the building lease as an operating lease. If upon completion of construction, the project does not meet the “sale-leaseback” criteria, the leased property will be treated as a capital lease and included in building and building improvements in the table above.            

In addition, as part of the SolarCity acquisition, we assumed a build-to-suit lease arrangement with the Research Foundation for the State University of New York, or the Foundation, for the construction located in Buffalo, New York. See Note 17, Commitment and contingencies, for Build-to-Suit lease arrangement with the Foundation.

As of December 31, 2016 and December 31, 2015, the table above includes $1.32 billion and $206.1 million of build-to-suit assets. As of December 31, 2016 and December 31, 2015, corresponding financing obligations of $3.8 million and $1.3 million are recorded in accrued liabilities and $1.3 billion and $201.3 million are recorded in other long-term liabilities.

Depreciation and amortization expense during the years ended December 31, 2016, 2015 and 2014 were $477.3 million, $278.7 million and $155.9 million. Total property and equipment assets under capital lease as of December 31, 2016 and 2015 were $112.6 million and $58.1 million. Accumulated depreciation related to assets under capital lease as of these dates were $40.2 million and $22.7 million.

We have incurred $825.3 million and $317.5 million of costs for our Gigafactory 1 as of December 31, 2016 and 2015.