All owners of taxable personal property are required, annually, to file a Personal Property Declaration Form M-15. A single personal property assessment is made for all taxable personal property of the owner located in the city or town. 2.If I move out of Middlesex County or sell an item of personal property, will my tax bill be prorated? Middlesex County does not prorate personal property. The COUNTY OF MIDDLESEX Commissioner's Office offers convenient and secure online search services. The tax balances on the online search system are maintained. Exemption applications must be filed with your local assessor's office. See our Municipal Profiles for your local assessor's mailing address. Property Tax Reimbursement Forms (PTR) May Be Filled Out on the Following Days and Times. Taxpayers should use these forms and guides to apply for local tax abatements and exemptions and file property returns.