To add your real property to the MyFairfax portal, you need the property's Tax Map Reference Number and the Web Control Number. PLEASE READ BEFORE FILLING OUT ON-LINE FORMS.Adobe Reader XI is required to be able to save a filled out copy of your filing for your records. Each year, the Real Estate Assessment Office appraises all real property in the City to determine its value for tax purposes. A One Time Showing Agreement is a commission agreement signed between a buyer's agent and a seller for specific named buyers. The instructions are also provided under Forms on this website. Do not use the signing option when filling out the forms. Fairfax County is not accepting electronic signatures at this time. The assessed value is the basis for your real estate taxes which will be due on June 21 and December 5. What is the assessment ratio in the City of Fairfax?