How do you calculate project costs?

We use current market rates for high-quality materials and skilled labour in Durham Region. Every estimate includes a detailed breakdown of tasks so you know exactly where your money is going.

Is the initial consultation free?

Yes. We offer a free first meeting to look at your space and talk about your goals. After that we provide a clear price range for your specific project.

Do you offer fixed-price contracts?

Most of our projects use fixed-price contracts. This means the price we agree on is the price you pay, unless you decide to change the scope of work later.

What happens if material prices go up?

Once we sign a contract, we lock in the material prices for you. We order supplies early to make sure your budget stays safe from market changes.

How long will my renovation take?

Small jobs might take two weeks, while large additions can take months. We give you a full schedule before we start so you can plan your life around the work.

Will I have a dedicated project manager?

Yes. You will have one main person to talk to. They will be on site often to check quality and keep you updated on our progress every day.

How do you keep the job site clean?

We clean up at the end of every single day. We use floor protection and dust barriers to keep the rest of your home safe and tidy while we work.

Do I need to move out during construction?

For most room renovations you can stay home. For very large projects like whole-home guts, we might suggest staying elsewhere for a short time for your comfort.

What areas do you serve?

We serve all of Durham Region including Oshawa, Whitby, Bowmanville, Ajax, Pickering, Newcastle, Clarington, Scarborough, Stouffville, and Peterborough.

Do you handle permits and inspections?

Yes. We submit the plans to the municipality, get the permits, and schedule all required inspections to ensure your home is safe and the work is done to code.

Can you help with the design phase?

Yes. We work with designers and architects to help turn your ideas into professional drawings before we start building.

Do you do commercial construction?

Yes. We offer commercial services for offices, retail spaces, and restaurants alongside our residential renovation work.

What kind of warranty do you provide?

Every Cornerstone project is backed by a 5-year written workmanship warranty. If something we built fails because of how it was installed, we fix it at no charge.

What if I find a problem after you leave?

Contact us directly. We respond to warranty concerns within 1 to 2 business days and will schedule a site visit to assess the issue at no cost to you.

Are you insured and WSIB compliant?

Yes. Cornerstone Construction carries full liability insurance and all our workers are covered by WSIB. This protects you and your home completely.

Do you offer maintenance tips?

Yes. At the end of every project you receive a full Owner's Manual with care and maintenance instructions for every material and system we installed.

Additions & Garages Oshawa · Additions & Garages

Home Additions & Garages in Oshawa

Oshawa's older neighbourhoods mean narrower lots, mixed zoning, and rules that change block to block. Before we design anything, we check what your specific zone actually allows — so the price we give you doesn't change once the City weighs in.

Start Your Construction Diagnostic Call 1-888-404-7383

Oshawa at a Glance

0.6mMin. setback for garages & accessory buildings (side & rear)

60m²Max ground floor area for an accessory building conversion (90m² in R1-G/R1-H)

2-4Weeks, typical permit review for a straightforward addition

4ftProperty-line distance that triggers fire-rated window protection

Before We Design Anything

The Rules That Actually Matter

Oshawa's zoning by-law (60-94) sets different setbacks and limits depending on your specific zone — R1, R2, R3, and several others each with their own rules. We pull your exact zone from the City's mapOshawa tool before we draw anything, so the number we give you holds.

Setbacks & Lot Coverage

An attached addition follows your base zone's side, rear, and front yard setbacks — these vary by zone, so we confirm yours before design, not after.
A detached garage or accessory building gets a reduced setback: 0.6m (2ft) from interior side and rear lot lines.
Converting an accessory building into living space caps out at 60m² of ground floor area (90m² in R1-G and R1-H zones).
Lots near hazard lands flagged by CLOCA (the conservation authority) carry extra restrictions — worth checking early if you're near a ravine or waterway.

What Needs a Permit

Additions, basement finishing, garages, decks, and any structural, plumbing, electrical, or HVAC change all require a building permit.
The City itself is upfront that review timeframes aren't guaranteed right now due to application volume — plan around real weeks, not a promise.
Straightforward additions typically clear review in 2 to 4 weeks; more complex or non-standard designs can run longer.
We track the application status and keep you updated — you're not the one calling Service Oshawa to check.

If your addition includes a basement or a secondary suite, see our legal basement apartment guide for Oshawa — the parking and unit-count rules there are separate from what applies to a straight addition or garage.

Building Up

What a Second-Storey Addition Actually Requires

Adding a full floor on top of your existing home isn't just framing walls — it changes the structure underneath it too.

What's Involved

Full roof removal down to the existing top plate before the new floor goes on.
A new engineered floor system spans where the roof used to be — this becomes your new upper floor.
The ceiling on your existing main floor gets disturbed by this work and needs refinishing once the new floor system is in.
Structural engineering confirms your existing walls and foundation can actually carry the new load — not optional, required on every second-storey build, and covered under our BCIN design & permit drawing services.
Any new window within 4 feet of a property line needs 45-minute fire-rated protection under Oshawa's rules — an easy detail to miss on narrower lots, and one we check before design.

See real pricing for second-storey additions in our investment calculator

How It Works

Zoned, Designed, Permitted, Built

An addition isn't just built — it's checked against your zone, engineered, permitted, and inspected. We carry all of it.

Confirm Your Zone

Pulled from mapOshawa — your exact setbacks, lot coverage, and any CLOCA hazard-land considerations, checked before we design.

Design & Engineered Drawings

Scaled plans plus structural engineering where required — mandatory for any second-storey work.

Building Permit

Filed with the City. Straightforward additions typically clear in 2 to 4 weeks; we track it so you don't have to.

Build & Inspections

Framing, structural, electrical, and plumbing inspections through to final sign-off.

$300

Our on-time guarantee. If we miss the agreed completion date for reasons within our control, you're compensated $300 for every day past it — part of the Certainty Stack on every Cornerstone build.

Why Build It Permitted

A Permitted Addition Is Worth More

Resale Confidence

A permitted addition shows clean on a title search. An unpermitted one can hold up your sale or force disclosure to buyers.

Insurance Stays Valid

Unpermitted structural work can void your coverage if something goes wrong down the line — permitted work protects your policy.

No Forced Teardown Risk

The City can order unpermitted additions removed or brought up to code after the fact. Done right the first time, that risk doesn't exist.

Built to Carry the Load

Proper engineering means your addition holds up for decades — not just long enough to pass a first inspection.

Common Questions

Oshawa Addition & Garage FAQ

Do I need a permit for a home addition in Oshawa?

Yes. Additions, basement finishing, garages, decks, and any structural, plumbing, electrical, or HVAC change all require a building permit through the City of Oshawa. We handle the application and coordinate every inspection.

What's the setback if I want to build a detached garage?

A detached garage or other accessory building needs a minimum 0.6m (2ft) setback from interior side and rear lot lines. Ground floor area caps at 60m² for a living-space conversion (90m² in R1-G and R1-H zones). Your specific zone can carry additional requirements, which we confirm before designing.

How long does an Oshawa building permit actually take?

Straightforward additions typically clear review in 2 to 4 weeks. The City itself has stated it can't guarantee review timeframes right now due to application volume, so more complex or non-standard designs can take longer. We build realistic timelines into your project plan rather than a best-case guess.

What's this fire-rating rule for windows near my property line?

If a window in your addition sits within 4 feet of a property line, Oshawa's building code requires 45-minute fire-rated protection on that opening. It's an easy detail to miss and an easy one to get flagged on inspection — we check lot setbacks and window placement before we ever draw up a plan.

Can I convert my existing garage into living space instead of building new?

Yes — this falls under Space Conversion rather than new construction, since there's no new foundation or exterior envelope to build. It's a genuinely different scope and cost than a new addition or a new garage build.

What happens if my addition doesn't fit my zone's setback rules?

If your lot falls just short of a setback or coverage rule, a minor variance through the Committee of Adjustment can often still make the project work. We flag this early rather than after drawings are done, so you know your real options before committing to a design.

Zoning and permit information last reviewed September 2026. Municipal requirements can change — Cornerstone confirms current City of Oshawa requirements with the building department on every project. Sources: City of Oshawa Zoning By-law 60-94; City of Oshawa Building Permits and Accessory Apartments pages.

Part of Our Durham Additions & Garages Guide

See our full investment calculator, garage attached vs. detached breakdown, and FAQ

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Also building additions and garages in Whitby, Ajax, and across Durham Region.