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Adding a Second Unit to an Existing House

Adding a second unit is a great way to provide independent living for a family member or make a little extra money to help pay off the mortgage. But there are things you need to consider as you decide whether to build a second unit. Is your home detached, semi-detached or a rowhouse that is more than five years old? Are you considering renovating it to add a second unit? If the answers are yes, then this guide can help you. It fulfills a commitment made in Ontario’s Housing Supply Action Plan to provide homeowners with a user-friendly guide and checklist to help you build legal second units.

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Room Sizes and Floor Area

The Building Code sets out minimum room sizes in dwelling units, including second units. Room sizes vary depending on whether rooms are separated by walls or the unit is open concept. For example, the minimum area of a dining room separated by walls is 7 m2 (75 ft2), while an open concept bachelor unit combining sleeping, living, and dining areas and kitchen space is permitted to be 13.5 m2 (145 ft2).

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Ceiling Heights

You are also required to have minimum ceiling heights in your rooms. Meeting the ceiling height requirements can sometimes be a challenge in an existing house. Choose the location of your second unit carefully. Ceiling height requirements for second units vary in different parts of your house such as in basements and attics.

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Windows

Windows are required in a second unit. The size of windows you need is determined by the size of the unit. Larger units will require more or bigger windows. Window sizes are also based on what type of room they are in. Some windows can also be used as another way to exit a second unit.

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Plumbing

Your second unit will have to meet plumbing requirements. At minimum you will need:

  • A hot and cold water supply
  • A sink, bathtub or shower, toilet, or a drainless composting toilet in the bathroom
  • A kitchen sink
  • Access to laundry facilities, which may be provided in a shared laundry room or a separate laundry area in the second unit.
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Heating and Ventilation

The Building Code allows a house with a second unit to have a single furnace and common system of air ducts. For fire safety, you will have to install a special type of smoke detector in the main supply or return air ducts. When activated, this device turns off the fuel supply and electrical power to the furnace causing it to shut down and preventing the spread of smoke from one unit to the other.

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Electrical Facilities and Lighting

To ensure adequate lighting in rooms and spaces, the Building Code requires that your second unit must have a light and switch in every room and space of your second unit. A switch at both the top and bottom of the stairs.

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Smoke Alarms

You will need smoke alarms (sometimes known as smoke detectors) in your second unit that must meet the CAN/ULC S531 performance standard. A label on the alarm will indicate whether it meets this standard. Your smoke alarms must also have a flashing light when they are activated. Smoke alarms are widely available at hardware and home renovation stores.

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Exits

Your second unit needs safe exits. There are different rules for exits depending on where in your house the second unit is located. Providing a separate exit for the second unit is preferable.

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