Hi and welcome to Just Answer!1)Why did the county increase my property's assessed value - and, thus, my property taxes? Because of new construction the value of the property was reassessed. Subsequently, real property is reappraised for tax purposes when new construction is completed.If you think that assessment is incorrect - you may appeal. See more information here - https://lacaab.lacounty.gov/pdf_forms/AssessmentAppealsHowToPrepareBrochure.pdf and here - http://www.boe.ca.gov/proptaxes/pdf/pub30.pdf 2)Doesn't Proposition 13 come into play here? California’s system of property taxation under Article XIIIA of the state Constitution, commonly referred to as Proposition 13, values property at its 1975 fair market value with annual increases limited to the inflation rate, as measured by the California Consumer Price Index, or 2%, whichever is less.Under certain conditions, persons aged 55 and older or severely disabled persons of any age may transfer the Proposition 13 factored base year value of their principal residence to a residence acquired or built as its replacement. If you qualify - you need to file an application within your county. 3)Does it not matter that no square footage was added?It does matter for determine the value of your property, but because county assessors must annually assess all taxable property in the county - that fact may not prevent reassessment.