What is an Additional Residential Unit (ARU)?

An ARU is a self-contained residential unit with a private kitchen, bathroom facilities and sleeping areas within dwellings or within structures ancillary to a dwelling. Additional Residential Units are also referred to as second units, basement apartments, accessory apartments, granny flats, in-law apartments or nanny suites

Why Additional Residential Units?

The City of Peterborough is permitting the creation of ARUs to achieve the following objectives:

• To provide housing for a variety of users including aging family members, adult children, caregivers etc.
• To increase the supply of rental housing and improve rental affordability
• To alleviate the cost of homeownership
• To make efficient use of infrastructure, housing stock, land and resources
• To facilitate residential intensification, mitigate climate change, and reduce greenhouse gas emissions

Building a new Additional Residential Unit

Please visit our Building a New Additional Residential Unit page to see Zoning By-law requirements associated with new ARUs.

Existing Additional Residential Units

If you have an existing ARU, please visit our Legalizing Existing Additional Residential Units page to ensure your ARU meets applicable regulations.

Obtaining a Building Permit

If your property meets requirements for an ARU, please see the requirements for building permit application at the bottom of this page.

Obtaining a building permit

Please be advised; additions, new construction or changes to occupancy may result in changes to property valuation. Please contact Municipal Property Assessment Corporation for further information, or contact Service Peterborough at 705-742-7777 ext. 1865 to discuss any possible tax implications resulting from a change to your property.

The information below is intended to familiarize homeowners and their contactors with general requirements set out by the Ontario Building Code, and applicable law to help assist in obtaining a building permit for construction of new or conformity of existing Additional Residential Units.

Sections below outline general submission requirements such as types of documents and drawings necessary for review of an Additional Residential Unit construction, as well as comprehensive requirements applicable to the construction of living spaces and safety requirements such as fire prevention.

Permit applications for Additional Residential Units are to be submitted through the online application portal.

Submission Requirements

1. Completed Building Permit Application Form – generated in the portal to be signed.

2. Drawings/plans drawn to scale and fully dimensioned either in metric or imperial but not in combination. Drawings must include:
   • Site Plan
   • Existing Floor Layouts
   • Proposed Floor Layouts
   • Cross Sections and Wall/Ceiling/Fire Separation Construction Details
   • Elevations showing ARU Entrance and Egress Window
   • HVAC and mechanical drawings

The site plan and lot grading and drainage plan must clearly indicate an unobstructed and maintained path for firefighting access that meets the requirements of Section 6.33.3(d) of the City’s Zoning By-law. This must be provided to and approved by Engineering Development before permit may be granted. 

Sample drawings can be found at the bottom of the page.

If constructing an ARU in an ancillary/accessory building, proof that the existing sanitary and domestic cold water (DCW) services will adequately support the increased load. An engineer possessing PEO designation will be required to verify/design the plumbing systems to ensure they meet the OBC. Only one sanitary and DCW service from the City services is permitted per property.

Who can Prepare Drawings?

Drawings are to be prepared by a professional such as an engineer, architect, or a designer with a Building Code Identification Number (BCIN). 

1. If there is a Principal Unit with one additional residential unit within the dwelling, the homeowner may be permitted to prepare the drawings. It is recommended that the owner engage the services a BCIN designer qualified with a minimum House

2. If there are a total of three (3) dwelling units within the dwelling, a designer with minimum qualifications in Small Buildings is required.

New Ontario Building Code Clarification

Note that the Ontario Building Code 2024 added the following definition.

Secondary suite means a self-contained dwelling unit located in a building or portion of a building of only residential occupancy that contains only one other dwelling unit and common spaces, and where both dwelling units constitute a single real estate entity.

Secondary suites have special exceptions under the OBC. If you are proposing two additional residential units in one dwelling, the provisions for a secondary suite in the Code do not apply.

There is a new portion in the OBC Appendix to assist in clarifying secondary suite provisions from a technical perspective.

Secondary Suite (A-1.4.1.2.(1))

A secondary suite is a self-contained dwelling unit of a prescribed maximum total floor area that is part of a building containing not more than two dwelling units (including the secondary suite) and any common spaces such as common storage, common service rooms, common laundry facilities or common areas used for egress. Secondary suites are typically created within an existing single dwelling unit—commonly called a "house"—either constructed as an addition to an existing house or incorporated during the construction of a new house. A secondary suite may have more than one storey and may be on the same level as the principal dwelling unit of the house or be above or below it.

Examples of buildings where secondary suites are permitted include individual detached houses, or where the secondary suite is located in a portion of a building, semi-detached houses (half of a double) and freehold row houses.

Where a building has multiple occupancies, the secondary suite can only be created in a portion of the building that is of residential occupancy. Apartment buildings have more than two dwelling units and are therefore not permitted to have secondary suites.

Neither dwelling unit in a house can be strata-titled or otherwise subdivided from the remainder of the house under provincial or territorial legislation. This means that both dwelling units are registered under the same title.

Secondary suites are also referred to as "accessory suites" or "secondary units" by various jurisdictions.

The allowances in the OBC for secondary suites only apply where there are not more than two dwelling units in the house; they would not apply to a second ARU (i.e. third dwelling unit).

General Design Requirements

• Carbon Monoxide alarms are required adjacent to each sleeping area
• Each unit shall have access to common laundry facilities or have connections within each unit for the connection of laundry appliances
• Minimum required dwelling unit size is 30 m2 and meet minimum room sizes as per Ontario Building Code requirements

Note: This design advice does not cover all requirements for ARUs.  It is the designer’s responsibility to ensure compliance with the current Ontario Building Code.  A copy of the Ontario Building Code (Ontario Regulation 332/12 made under the Building Code Act, 1992) is available online through the e-Laws website.