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.

How long does a typical addition take from permit to completion?

Permit review alone can run 3-8 weeks depending on your municipality. Construction timelines vary widely by scope. We give you a real, written timeline as part of your project diagnostic, backed by our $300/day on-time guarantee.

Can I add a garage without extending my driveway?

Depends on your municipality's driveway width and lot coverage rules. We check this as part of your zoning review before design begins.

Is a sunroom or bump-out cheaper than a full addition?

Generally yes on a total-cost basis, though per-square-foot small additions can run higher since fixed costs like permits spread across fewer square feet. Our project diagnostic gives you a real number for your specific size and scope.

Built to Last

Additions That Look Like They Were Always There

A new addition isn't a bolt-on. It's structural work that has to tie into your home, carry real loads, and pass inspection. We bring commercial-grade engineering and discipline to every addition, garage, and second storey we build.

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

Back to Residential Services

35+Years Experience

OBCCompliant Builds

100%Permit Managed

5yrWorkmanship Warranty

More Than Four Walls

We Build Additions the Right Way — From the Foundation Up

Most additions fail in the parts you can't see. A footing that wasn't dug deep enough. A beam that wasn't sized for the load. A roofline that doesn't drain. A connection to the existing house that lets water in two winters later.

We come from a commercial and structural background, so we treat your addition like what it is: a permitted, engineered build that becomes part of your home forever. New foundations, engineered beams, proper rooflines, and a finished result that looks like it was always part of the house — not stuck on the side of it.

Second-Storey Addition

A Full Floor, Added On Top

A complete second storey framed over an existing main floor. Crane-set framing, engineered to carry the new load — doubling the home's living space without moving.

Rear home addition with a new roofline tied into the existing house

Rear Addition & New Roofline

Built Into the Existing Home

A rear addition integrated into the existing home, with a new roofline tied cleanly into the original structure. High-performance exterior sheathing and a poured foundation built for our climate.

Garage Build

Engineered, Not Just Assembled

A new garage built on a proper foundation with an engineered beam header. Framed, sheathed, and squared — built to the same standard as the house it sits beside.

Our Addition Services

From a Single Room to a Second Storey

Second-Storey Additions

Add a full floor of living space without giving up your lot or your location.

Rear & Side Additions

Extend your main floor for a bigger kitchen, family room, or primary suite.

In-Law & Family Suite Additions

Self-contained additions built for comfortable multi-generational living.

Attached & Detached Garages

Engineered, permitted garages built to match the look of your home.

Space Conversions

Turning an existing garage or attic into finished, livable room — no new foundation required.

Foundation & Structural Work

New footings, foundations, and engineered beams to carry it all.

Before We Design Anything

What Your Municipality Allows

Every Durham Region municipality runs its own zoning bylaw — setbacks, lot coverage limits, maximum building height, and driveway or garage width rules all vary town to town. What's allowed in Whitby isn't automatically allowed in Ajax. We check your property's specific zoning before we design anything, not after.

Oshawa

Side/rear setbacks · max lot coverage · height limits on second-storey additions

VIEW OSHAWA REQUIREMENTS

Whitby

Zoning bylaw setbacks · garage width limits · driveway coverage rules

VIEW WHITBY REQUIREMENTS

Ajax

Lot dimension standards · parking requirements · building height limits

VIEW AJAX REQUIREMENTS

Pickering

$165 min. permit fee · zoning pre-review recommended · 4-8 week review

VIEW PICKERING REQUIREMENTS

Clarington

Bowmanville · Courtice · Newcastle — zoning varies by area, up to 2 ADUs on urban lots

VIEW CLARINGTON REQUIREMENTS

Uxbridge

Rural zoning · septic capacity for plumbing additions · larger setback variances

VIEW UXBRIDGE REQUIREMENTS

We pull your property's zoning designation before quoting, so the number we give you accounts for what your specific lot can legally support — not a generic estimate that changes once the city weighs in.

Garage Builds

Attached vs. Detached

Attached Garage

Ties directly into your home's existing structure — shared wall, often shared roofline. Requires careful fire separation between the garage and living space, which the Ontario Building Code requires regardless of municipality, and coordination with your existing foundation.

Detached Garage

A standalone structure with its own foundation and roof. More design freedom, but still subject to your municipality's setback rules from property lines and distance-from-house requirements. Larger detached garages typically require a full building permit, not just a minor structure exemption.

What It Costs

What Does it Cost to Build an Addition, Garage or a Space Conversion in Durham Region?

Select your project type, size, and scope below for a real investment range. Your exact number depends on your site, your municipality's zoning, and how the addition ties into your existing home — but this gets you close before we ever walk your property.

Addition & Garage Investment Calculator

Select your project type and customize the details to see a real estimate of your investment.

Project Type

Square footage400 sq ft

200 sq ft800 sq ft

Finish level

Adding a second floor too?

Basement Below

Kitchen

Exterior Finish Match

Estimated investment range

$135,000 – $175,000

Ground-Floor Addition

This is a ballpark figure, not a quote. Every property and scope is different — treat this as a starting point for the conversation, not a number to build a budget on.

Included in this price: permits, engineered drawings, and standard heating/cooling extension for the new space. These aren't add-ons — every addition needs them, so they're built into the per-square-foot rate above.

Ranges reflect general Durham Region market pricing as of 2026 and do not account for site-specific conditions, existing structural issues, or municipal zoning restrictions on your property. Your actual investment is confirmed only after a site visit and written proposal — price locked before work begins, not before we've seen your home.

Start Your Addition Diagnostic

The Cornerstone Difference

Engineered. Permitted. Built to the Commercial Standard.

Engineered Drawings & Permits

We handle the structural drawings and the permit process, so your addition is legal, safe, and inspection-ready — see our BCIN Design & Permit Drawing Services.

Structural Discipline

A commercial and ICI background means we size beams, footings, and connections to last for decades — not just pass the first look.

The Certainty Stack

Price Lock Guarantee, $300/day on-time guarantee, a real-time client portal, daily cleanup, and a 5-year warranty on every build. Learn more →

A Seamless Tie-In

We match rooflines, brick, and finishes so your addition looks like it was always part of the home.

Before You Build

An addition is exactly the kind of project where the building needs its own coverage. Learn about builder's risk insurance for your addition.

Common Questions

Additions & Garages — What Homeowners Ask

Yes. Any addition that adds livable square footage — a room, a second storey, a garage — requires a building permit in every Durham Region municipality. Setbacks, lot coverage limits, and permit fees vary by town, which is why we check your specific municipal zoning before quoting, not after.

A rear or side addition extends your main floor outward, on a new foundation. A second-storey addition builds up instead of out — no new foundation, but it does require removing the existing roof, installing a new floor system for the upper level, and refinishing the ceiling on your existing main floor below. Second-storey work generally costs more per square foot because of that structural scope.

Permit review alone can run 3-8 weeks depending on your municipality. Construction timelines vary widely by scope — a small bump-out is a different build than a full second storey. We give you a real, written timeline as part of your project diagnostic, backed by our $300/day on-time guarantee.

We handle it. Engineered drawings, permit applications, and zoning review are all part of your project — see our BCIN Design & Permit Drawing Services for how that process works. You don't need to navigate your municipality's building department on your own.

Depends on your municipality's driveway width and lot coverage rules — some towns tie garage size directly to required driveway access. We check this as part of your zoning review before design begins, so you know what's actually allowed on your property before we draw anything up.

Opening up an existing wall or roofline can occasionally reveal something a visual inspection couldn't catch — outdated wiring, past water damage, an undersized structural member. We flag it immediately, explain what we're seeing and why it matters, and get your sign-off before any change in scope or cost. No surprise invoices.

Generally yes, on a total-cost basis — a smaller footprint means less foundation, less roofline, less finishing. Per-square-foot, small additions can actually run higher than larger ones since fixed costs like permits and mobilization spread across fewer square feet. Our project diagnostic gives you a real number for your specific size and scope.

Every addition we build is covered by our 5-year workmanship warranty as part of the Certainty Stack — the same guarantee that applies across all our residential work. See our Certainty Stack page for the full details on what's covered.

Yes — a ground-floor addition can be built with a full basement, a crawlspace, or on a standard slab, depending on your site conditions and what you want out of the space below. A finished, usable basement under an addition costs more than leaving it unfinished, but it's often worth it if you're already opening up the foundation.

You're covered by our $300/day on-time guarantee, part of the Certainty Stack. If we miss the agreed completion date for reasons within our control, you're compensated for every day past it — no chasing us for it, it's built into the agreement upfront.

Thinking About Adding On?

Book a free project diagnostic. We'll walk your home, talk through what's possible, and show you exactly how we'd build it.