3.6K 
A technical change beneath Toronto’s streets has created a roadblock for new housing. Since early 2022, the City’s Foundation Drainage Policy has required most new buildings to use watertight (or “bathtubbed”) basements, banning the small groundwater discharges that once kept basements dry. Meant to protect sewers, the rule has added millions in construction costs and thousands of tonnes of embodied carbon to projects citywide, particularly mid-rise, affordable, and non-profit housing. A team of engineers led by Grounded Engineering is now proposing “low-flow basements,” a new concept that could exempt some projects and help unlock stalled housing without overwhelming city infrastructure.
Toronto Water introduced the Foundation Drainage Policy on January 1, 2022, to preserve sewer capacity amid heavier storms and flooding. The new rule prohibits any new foundation drainage into municipal systems, so basements can no longer include weepers or subfloor drains to collect groundwater. Instead, every structure encountering the water table must be sealed to resist groundwater pressure. The change effectively replaced conventional spread footings with full raft foundations acting as waterproof tanks, a design once reserved for subway stations. As the City views the new Foundation Drainage Policy as merely enforcing the existing Sewer By-law, they did not need or seek Council approval, even though it introduced major changes to the status quo. It applies citywide, regardless of conditions.
Replacing shallow spread footings with raft foundations increases concrete, steel, excavation, and shoring, raising both costs and carbon emissions. For a typical six-storey mid-rise with one basement, that translates to an added $1.5 to $3 million, and up to a 25% increase in embodied carbon. These premiums are levied on every basement, not just high-rise towers. Affordable housing and non-profit housing projects such as the West Neighbourhood House on Ossington Avenue and Kindred Works’ church redevelopments are also billed, adding even more financial burden. The policy also states that financial hardship cannot be cited for exemption, leaving smaller projects (including shallow, low-risk sites) facing the same engineering demands as a high-rise tower or a subway station.
Grounded Engineering, along with a team of engineers including Jablonsky, RJC, and Smith and Andersen, has submitted a proposal to the City to exempt low-flow basements from the watertight rule. These are basements that would discharge less than 50,000 litres per day (roughly the flow of a single garden hose) into municipal sewers. That threshold mirrors the Ministry of the Environment’s limit for water takings that do not require provincial permits, making it a reasonable starting point. In practice, many mid-rise and single-level basements fit within this range.
The concept works within the existing framework. Section 5 of the Foundation Drainage Policy already allows exemptions, though current criteria exclude cost as a factor. This low-flow approach provides a clear, measurable standard where infrastructure impact is minimal. Qualifying projects could use conventional drained foundations with perimeter weepers and subfloor drains without triggering the financial burden of full watertight design.
In practice, low-flow basements would be regulated through engineering data. Each project would submit engineering reports confirming discharge rates and local sewer capacity. Smart meters could then monitor flow volumes in real time. City officials are exploring whether thresholds should be per resident, per square metre, or tied to affordability. Under a motion adopted by Councillor Josh Matlow in July, 2025, Toronto Water are modelling these scenarios to determine how low-flow allowances could be safely implemented.
Watertight basements increase embodied carbon by as much as a quarter on a typical mid-rise, adding thousands of tonnes of concrete and steel that run counter to the City’s own Toronto Green Standard. A low-flow exemption would reduce material use while still protecting sewers through measurable discharge limits.
While watertight basements may sound like the safer option, they often create long-term risks for owners. Tiedown anchors used to secure heavy raft slabs pierce the waterproofing membrane, which will eventually leak. Once occupied, repairing a foundation several storeys underground is nearly impossible, leaving condo boards with costly maintenance. The Deep Foundations Institute advised the City to require emergency drainage systems as a safeguard, but the City only started approving these after extensive lobbying from the team of engineers led by Grounded. Without built-in weepers or sump connections, even minor leaks can flood parking, and later pump retrofits only partly help.
Momentum is building after Councillor Josh Matlow’s motion directing staff to study the technical and policy framework needed to allow low-flow basement exemptions. Senior staffers at Toronto Water and the City Manager's Office have shown interest in the proposal. Toronto Water, meanwhile, is developing modelling tools to test potential thresholds and enforcement methods, though it currently has no formal process for granting exemptions.
By treating all groundwater discharge as equally problematic, the current policy has delayed many housing projects. A low-flow exemption could reopen those opportunities, especially mid-rise and community-based projects with single basement levels. The approach keeps full watertight standards in place for high-flow conditions, such as the groundwater-heavy soils of Scarborough, ensuring that environmental protection remains intact and City sewer capacity is protected. For a city aiming to meet housing and climate goals, this may be one of the simplest, most impactful fixes.
* * *
UrbanToronto's research and data service, UTPro, provides comprehensive data on construction projects in the Greater Golden Horseshoe—from proposal through to completion. Other services include Instant Reports, downloadable snapshots based on location, and a daily subscription newsletter, New Development Insider, that tracks projects from initial application.
