What you need to know about septic systems before purchasing a building lot in Ontario

Planning to purchase a
lot to build a home in Ontario?

Before you do anything else, check if a public sewer system is available? If not, you will need a septic system to treat and dispose of sewage from the home you plan to build.

What type of septic system you will need, how it will be designed and constructed depends on the particular lot, how much space you have, the characteristics of the surrounding land and the make-up of the soil.

Whatever type, all septic systems require careful attention to design, construction, operation, and maintenance.

The septic system typically consists of a septic tank buried in the ground and a drain-field that can fit within the front or back yard of the home-site.

Septic system

Septic system

Household discharge from the kitchen, bathrooms, and laundry room flow directly into the septic tank where the solids are retained and separated from the liquid.

The liquid waste flows out of the tank through a series of pipes to where it is slowly released into the leaching field where it leaches through the soil and is purified before reaching the groundwater.

The leaching bed is made up of porous materials, such as sand and gravel, and acts as a filter to clean the water before it seeps back into the ground.


Light solids, such as soap suds and fat, float to the top and form a scum layer. This layer remains on top and gradually thickens until you have the tank cleaned.

Unfortunately, not all soils can absorb wastewater or purify it.

Septic systems that are installed in unsuitable soils usually malfunction by leaking raw, untreated sewage to the surface of the ground or a roadside ditch, or by contaminating the groundwater. The sewage may contain deadly bacteria and viruses.

It can be expensive to remedy the odor problems and potential health hazards that result from the use of septic systems in unsuitable soil.

Because of that, Ontario Building Code requires an expansive soil and site assessment by the local health department to determine the suitability of the soils and topography of the lot

Reviewing the Lot

If a septic system is needed, walk over the lot and look for indications of soil problems or site limitations that could affect the performance of a septic system.

Consider the following before you purchase a lot:

  • Is there enough space on the lot for the home, the septic system, and water well (if needed)?
  • Is the land next to a stream or river that could flood it?
  • Does the area seem wet and swampy?
  • Is there a bedrock that can interfere with the septic system?
  • Determine the type of soil on the lot by digging a two 5 foot holes where your field is to be situated
  • Find out what type of septic system, if any, will work on those soils.
  • Obtain a cost estimate for installing the kind of septic system you need.
  • Ask about the operating requirements and maintenance costs for the system.
  • Your well and all neighbors’ wells should be 100 feet or further from the septic system.
  • There must also be enough land for a “repair area” that can be used if the system needs expansion or replacement in the future.

Septic Systems in Ontario

Part 8: “Sewage Systems” of the Ontario Building Code (OBC) regulates the design, construction, operation and maintenance of on-site septic systems for most single-family homes.

In most areas, the local municipality’s Building Department examines plans, issues permits, and does inspections for systems regulated under the OBC. In some regions, this approval responsibility has been delegated to local Conservation Authorities or Health Units. The Municipal Building Department will be able to redirect inquiries.

Proper approvals under the OBC must be obtained by the homeowner before installing any of the suggestions for improving system performance.

Capacity of Septic System

The two primary elements that command the size and complexity of a system in Ontario are the maximum amount of waste water that the building could produce on a daily basis, and soil/site conditions.

Case in point; a small one bedroom /one bathroom house would have a maximum daily flow rate of about 750 liters. If that system were being located in soils with high absorption rate, then the system could be quite small and be installed at a reasonable cost.

On the other hand, if it is a huge five bedrooms home with a maximum daily flow rate of 2500 Liters and clay soil (which can only absorb 4 liters, per square meter, per day) then the cost could be enormous because a lot of sand would need to be trucked in.

It is almost impossible to determine how much a septic system is going to cost without visiting the property to assess the available space, access for large equipment, cleanup needs, etc.

Types of Septic Systems

The kinds of soil and site conditions on the land determine whether the local health department can issue a septic permit, as well as the type of septic system needed there.The conventional septic system, with a septic tank and a number of trenches buried 2 to 3 feet deep, is used at almost one-half of the homesites with septic systems in Ontario. It works well in brightly colored (red or brown), thick, loamy-textured soils with deep water tables. This type of system is relatively inexpensive; the average installation cost ranges from $7,500 to $10,000. On some soils that are too wet or too shallow for a conventional septic system, a modified standard system or an alternative septic system may be used.

Advanced Treatment Systems – Alternative Septic System

If a conventional system cannot be used, you may wish to determine the installation cost and maintenance costs of the alternative septic system that can be used.

Advantages of alternative systems:

  • Used on sites not suited for conventional septic systems
  • Need much smaller septic fields
  • Have the potential to remove significantly more bacteria and organic material than a conventional septic system
  • May extend the life of an existing leaching bed
  • Take up less room in the yard
  • May reduce nutrient output (depending on type)


  • More expensive to purchase and install
  • Are more costly to operate than a conventional septic system (electrical costs, media replacement)
  • Includes more mechanical parts that can break down or need replacement
  • Requires mandatory maintenance (increases costs)

You may want to consider alternative septic systems when:

  • Coping with lots with inadequate conditions for conventional systems
  • Dealing with lots that can’t accommodate the size of a regular bed
  • Replacing an old failed septic system
  • Building on hard-to-access lots where transporting materials for conventional systems is costly and difficult
  • If you want to provide additional nitrate reduction that some of the advanced treatment systems provide

There are many soils that are not suitable for an alternative septic system either.

For these reasons, it is in your best interest to determine the suitability of a lot for a particular kind of septic system before purchasing the lot. If you can use a conventional system, ask about any modifications to the system or to the site that may increase the installation cost.

Also, ask whether the approval affects the number of bedrooms that can be built in the home, or the location of the house, driveway, or a swimming pool.

In any case, it is a good idea to make the purchase of a building lot conditional upon the issuance of a permit for a particular type of septic system.