Additions to a House

A guide to prepare your building permit application to build an addition on a house.

An addition is defined as adding to an existing building. If you plan an addition that requires substantial removal of the existing house, you will require a separate demolition permit.

If you plan to create a separate dwelling unit, please refer to the accessory apartment page. 

Before preparing your permit

Some approvals and permits must be in place before you start preparing your building permit application.

Contact Ontario One Call at 1-800-400-2255 before you dig to locate underground utilities and avoid damage during construction.

For electrical work, contact the Electrical Safety Authority (ESA) at 1-877-372-7233 or Learn more at the ESA website

Project preparation

Detailed information is needed to properly evaluate your plans and the unique conditions of your property: 

Before you get started, consult the zoning by-law to ensure your house plans, proposed dimensions and location are permitted under the Town of Oakville's zoning by-laws which regulate the use, shape and location of buildings on properties. 

Visit our Zoning page or contact us at 905-845-6601 ext 3955.

If you plan to add additional plumbing fixtures you are required to submit a water service calculation form to verify your water service is adequately sized to handle the additional load or if upgrading is required.

If your addition adds a sixth or more bedroom, you will require a Building Code Part 6 ventilation design.

Heritage properties are managed to conserve important cultural heritage attributes. If your property is on the town's heritage registry it may require heritage approval. Learn more at on our Heritage Planning page or contact a town Heritage Planner for assistance at 905-845-6601 ext . 3870 or 3875.

If your project includes excavation and your property backs onto green space, a river bed or wetland you will need Conservation Halton approval. 

Learn more on Conservation Halton's website. Contact Conservation Halton for approval at or 905-336-1158.

Trees on your property, your neighbour's property and on the municipal right of way may require tree protection, particularly when heavy equipment and excavation is involved. In certain instances, a permit may be required to remove private trees. 

Learn more about tree removal and how to find a licensed arborist.

Other considerations

New construction or physical changes to your building or land may result in additional property taxes being levied.  These changes may result in an increase of value to your property, and the town is legislated to issue additional property tax billings due to an increase in value. 

  • The Municipal Property Assessment Corporation (MPAC) can provide the Town with increased property values (supplementary and/or omitted assessment) for the current tax year and two years prior when: 
    • land is omitted from the assessment roll 
    • use of the property changes 
    • property is improved due to construction, an addition, or renovation 
  • You will receive a Property Assessment Change Notice from MPAC, informing you of the supplementary and/or omitted assessment value. 
  • Supplementary and/or omitted residential property tax bills are calculated by multiplying the supplementary and/or omitted assessment value by the tax rate for the applicable tax year. 
  • MPAC has authority to assign omitted and/or supplementary assessment for the current year and two years prior. 
  • If you own a new property or have purchased a home in a new subdivision, the first regular bill you receive from the Town may only be for the portion relating to land value. You should expect a supplementary and/or omitted bill for the structure to follow. 
  • Supplementary and/or omitted assessments are issued under the authority of Section 34 and 33 of the Assessment Act. 

For more information on supplementary and omitted taxes, visit our Property Assessments & Appeals page or email