Basement Finishing & Renovation in Durham Region
Legal suites. In-law apartments. Luxury recreation spaces. Built to Ontario Building Code — with permits pulled and inspections passed.
Back to Residential ServicesWhat We Build Below Grade
Your basement is the most underused square footage in your home.
We build three types of basement projects at Cornerstone: legal income apartments registered under Durham Region bylaws, private in-law suites for aging parents or adult children, and luxury recreation spaces for families who want more room to live.
Every project is permitted. Every project is inspected. Every project is finished to the same Level 4 drywall standard we use on your main floor.
Choose Your Build Type
Every basement project starts with one question — how do you want to use the space? Each build type has different laws, different permits, and a different price range. Click through to learn exactly what applies to you.
Legal Second Suites & Income Apartments
Full compliance from design to occupancy permit. Fire separation, egress windows, OBC-compliant layouts, and interconnected alarms — all handled under one contract.
Learn MoreIn-Law & Family Suites
The stove rule. The bylaw consequences. The OBC requirements. We explain every tier — Guest Suite, Kitchenette, and Full In-Law Suite — before a dollar is spent.
Learn MoreRecreational & Entertainment Basements
Home theatres, wellness gyms, custom wet bars, wine rooms, golf simulators, and built-in millwork. We turn raw concrete into the space your family actually uses.
Learn MoreThe Cheapest Quote Is the Most Expensive Mistake You Can Make
In basement renovations — especially legal suites and in-law builds — a low quote almost always means one thing: the permit was skipped. Here is exactly what that costs you.
No Permit. No Protection.
An unpermitted basement suite can be shut down by the City of Oshawa with 24 hours notice. You lose the rental income. You may be ordered to gut the renovation you just paid for. Fines run from $500 to $100,000 under the Building Code Act — and each day of non-compliance is a separate offence. Your home insurance is voided the moment an unregistered self-contained unit exists in your home.
The contractor saved you $15,000. The fine cost you $50,000.
One Stove. Every Requirement.
The moment a stove is installed in a basement space, the Ontario Building Code classifies it as a self-contained dwelling unit — regardless of who lives there or whether money changes hands. That triggers a separate entrance, fire separation, egress windows in every bedroom, ESA electrical permits, and a full building permit. Contractors who quote low by skipping these items are not saving you money. They are transferring their risk onto you.
A kitchenette costs $8,000–$15,000. Getting caught with an unpermitted stove costs up to $100,000 — plus restoration, legal fees, and voided insurance.
Your Insurance Does Not Cover This.
Most home insurance policies are void or severely limited the moment an unregistered self-contained unit exists in your home. If there is a fire and an unpermitted stove or suite is discovered — your insurer denies the claim. You are personally liable for any injury, any damage, and any loss. The renovation you paid for is not covered. Your tenant is not covered. You are.
No permit — no coverage. It is that simple.
How to Spot a Low Bid That Will Cost You More
Every Cornerstone quote includes: permit coordination, BCIN-qualified drawings, ESA electrical, fixed price with a $300/day on-time penalty clause, and a 5-year workmanship warranty. That is what a real quote looks like.
Why Homeowners Choose Cornerstone Over Every Other Quote
Anyone can finish a basement. Not everyone can finish it legally, on time, on budget, and with a warranty that actually means something. Here is what separates us.
We Pull the Permit
Most basement contractors hand you a finished space and leave the permit to you. We pull every permit required — building, ESA electrical, and municipal registration — as part of the project. No permit means no protection. We never skip it.
We Explain Every Tier Before Quoting
Legal suite, kitchenette, or guest room — each tier has different laws, different costs, and different consequences. We explain exactly which tier applies to your situation before a number is discussed. No surprises after you sign.
$300/Day On-Time Penalty — In Writing
If we miss our completion date by even one day, $300 comes off your final invoice automatically. No arguments. No chasing us. It is written into every contract. Most contractors will not offer this because they know they cannot deliver.
BCIN-Qualified Drawings on Every Permitted Build
All drawings submitted for permit are prepared by a BCIN-qualified designer — not sketched on a napkin. Municipalities require this. Inspectors check this. It is the difference between a permit that gets approved and one that gets rejected.
ESA Electrical Coordinated as Part of the Project
The Electrical Safety Authority inspection is a separate legal requirement from the municipal building permit. Most contractors leave this to you. We coordinate the Licensed Electrical Contractor and ESA inspection as part of every permitted basement build.
Level 4 Drywall — Not Level 2
Level 4 drywall is the standard for painted walls that will be seen under direct or indirect lighting. Most basement contractors deliver Level 2 — you see every imperfection. We deliver Level 4 on every basement we finish regardless of budget.
Hospital-Grade HEPA Protection
Basement work means trades moving through your main floor every day. We run hospital-grade HEPA air scrubbers 24/7 and install fully sealed dust walls so the rest of your home stays clean and livable throughout the entire build. Your family does not live in a construction zone.
Daily Site Reset — Every Single Night
Every day before we leave, the last 30 minutes of our shift is dedicated to cleanup. Tools put away. Debris removed. Your home put back in order. You should not have to live in a disaster zone while we work. Most contractors leave the site the way it suits them — not you.
Real-Time Portal — You See Everything Daily
Log in anytime and see daily photos, financial tracking, and project updates. You never have to call us to find out what happened today. You already know. Most contractors go quiet after they cash your deposit. We stay visible.
5-Year Warranty + Owner's Manual at Handover
We stand behind our work for 5 full years — 5 times the industry standard. At handover you receive a complete Owner's Manual with every trade warranty, maintenance guide, model numbers, as-built photos, and a care guide for every surface. No other contractor does this.
Eight Guarantees. Every Project. In Writing.
Most contractors make promises. We put ours in the contract — and back them with our wallet. The Certainty Stack is what every Cornerstone client receives on every basement project regardless of budget.
Price Lock Guarantee
The number we quote is the number you pay. No surprises. No add-ons. No last-minute invoices that are double what you expected. What we put in the proposal is locked in — period.
$300/Day On-Time Penalty
We finish on time or we pay you. Literally. If we miss our completion date by even one day, $300 comes off your final invoice automatically. No questions. No arguments. No chasing us for it.
Hospital-Grade HEPA Protection
We run hospital-grade HEPA air scrubbers 24/7 and install fully sealed dust walls at every entry point. Your home stays clean and livable from day one to the last day of the build.
Real-Time Progress Portal
Log in anytime and see daily photos, financial tracking, and project updates — 24 hours a day, 7 days a week. You never have to call us to find out what is happening in your home.
Daily Site Reset
Every day before we leave, the last 30 minutes of our shift is dedicated to cleanup. Tools put away. Debris removed. Your home put back in order every single night.
5-Year Workmanship Warranty
We stand behind our work for 5 full years from the date of completion — 5 times longer than the industry standard. If something is not right, we come back and fix it. No cost. No excuses.
Owner's Manual at Handover
When we hand over your home, we hand over a complete binder with every trade warranty, maintenance guide, model numbers, as-built photos, and a care guide for every surface we touched.
Fully Licensed & Insured
Cornerstone Construction is a fully licensed and insured Ontario corporation. You are not hiring a guy with a truck. You are hiring a professional company that is legally accountable for every single thing we do in your home.
Price Lock — $300/Day Penalty — HEPA Protection — Real-Time Portal — Daily Reset — 5-Year Warranty — Owner's Manual — Licensed & Insured
"Most basements I walk into haven't changed in 30 years. Dropped ceiling tiles. Carpet over concrete. A bathroom nobody wants to use. We fix all of that — and we do it with permits in hand, not after the fact."
Inside a Cornerstone Basement
Legal Suite — Egress Window & Fire Separation
In-Law Suite — Full Kitchen Build
Home Theatre — Hidden Wiring & Acoustic Panels
Custom Wet Bar — Stone Counters
Accessible Bathroom — Curbless Shower
Open Concept Recreation Space
Every Cornerstone basement is built to Level 4 drywall standard with permits pulled, inspections passed, and a 5-year workmanship warranty at handover.
Everything You Need to Know Before You Build
Real questions from Durham Region homeowners. Real answers — no fluff, no sales pitch.
Do I need a permit for my basement renovation?
It depends on what you are building. A simple recreation room with no plumbing or structural changes may not require a permit. But any work involving electrical, plumbing, framing new walls, adding a bathroom, or creating a self-contained unit — including anything with a stove — requires a building permit from your municipality. Cornerstone assesses permit requirements at your project diagnostic before any work is scoped.
What is the difference between a legal suite and an illegal one?
The difference is a piece of paper called a final occupancy permit. A legal suite has been designed, permitted, inspected, and registered with the municipality. An illegal suite has not — and the consequences include fines up to $100,000, a forced closure order with as little as 24 hours notice, voided home insurance, and legal liability if a tenant is harmed.
Can I add a stove to my in-law suite?
Yes — but the moment you do, the Ontario Building Code classifies the space as a self-contained dwelling unit. That triggers a full set of requirements: separate entrance, 30-minute fire separation ceiling, egress windows in every bedroom, ESA electrical permit, and a full building permit. If you want a stove in the suite — and you should, because real independence means real cooking — we build the full permitted suite. We explain all three tiers at your diagnostic.
How long does a basement renovation take?
A standard basement recreation room takes 6–10 weeks from permit to handover. A legal suite or full in-law suite with plumbing, egress windows, and a separate entrance typically takes 10–16 weeks depending on permit processing times in your municipality. We build your completion date into the contract with a $300/day penalty clause if we miss it.
What is the minimum ceiling height for a legal suite in Oshawa?
The Ontario Building Code requires a minimum ceiling height of 1.95 metres (6 feet 5 inches) throughout all habitable spaces. Under beams and ducts the minimum is 1.85 metres (6 feet 1 inch). If your existing basement is below 1.95 metres, underpinning or bench footing is required before framing begins — we assess this at your diagnostic.
Does Cornerstone handle the permits or do I?
We handle everything. Building permit application, ESA electrical coordination, municipal registration where required, and the final occupancy inspection. You coordinate nothing. This is included in every permitted basement project we take on.
Can I legally rent out my basement after Cornerstone builds it?
Yes — if it is built as a full permitted legal suite with a registered occupancy permit and municipal registration where required. A recreation room, guest suite, or kitchenette suite cannot be legally rented. We explain exactly which build type qualifies for legal rental at your diagnostic.
What is included in the 5-year workmanship warranty?
The 5-year warranty covers all workmanship defects — anything we built that fails due to our labour or installation. It does not cover normal wear and tear, client-supplied materials, or damage caused by third parties. At handover you receive a complete Owner's Manual with every trade warranty, maintenance guide, and care instructions for every surface we finished.
How much does a basement renovation cost in Durham Region?
It depends entirely on the build type. A guest suite or recreation room starts at $30,000. A legal income suite runs $60,000–$110,000+. A full in-law suite with accessible bathroom and separate entrance runs $45,000–$120,000+. All pricing is confirmed in your written proposal with a locked price before work begins — no surprises after you sign.
What happens if Cornerstone misses the completion date?
$300 comes off your final invoice for every day we are late. Automatically. No questions. No arguments. This is written into every contract we sign. It is our way of putting our money where our mouth is on the on-time promise.
Ready to Do This Right?
Book your project diagnostic. We walk the basement, explain exactly which build type applies to your situation, and give you a written proposal with a locked price — before we touch a thing.
