Basement conversions are the cheapest ADU path in London at roughly $75,000–$150,000 over 2–4 months. Garage conversions run $80,000–$160,000 over 2–4 months. Detached units cost the most — $148,000–$350,000+ over 4–8 months — but deliver the highest privacy, the strongest rental appeal, and Tarion warranty coverage. Choose on lot access, budget and tenant type.
What are the main types of ADU in London, Ontario?
London homeowners have four ADU (additional residential unit, or ARU) paths, not three. Here is how they compare:
| ADU type | Typical cost | Typical timeline | Best for | |---|---|---|---| | Interior / basement conversion | $75,000 – $150,000 | 2–4 months | Lowest budget, no yard loss | | Garage conversion | $80,000 – $160,000 | 2–4 months | Detached feel at a lower cost | | Addition to existing house | $120,000 – $220,000 | 4–6 months | Multi-generational living | | Detached standalone unit | $148,000 – $350,000+ | 4–8 months | Maximum privacy and resale value |
These ranges reflect what Kuzyn Builds sees across London, St. Thomas, Strathroy, Sarnia and Grand Bend. The spread within each range comes almost entirely from three things: ceiling height and existing structure condition, distance to utility connections, and finish level.
Which ADU type is cheapest to build in London?
A basement conversion is almost always the cheapest, because the foundation, roof, walls and utility services already exist. You are paying for framing, insulation, egress windows, a bathroom, a kitchen, HVAC and finishes — not a new building envelope.
The cost killers to check before you commit:
- Ceiling height. Older London basements — particularly pre-1960 homes in Old South, Old North and Woodfield — often fall short of the height the Ontario Building Code requires for habitable space. Underpinning or lowering a floor can add more than the rest of the conversion combined.
- Egress. Every bedroom needs a compliant window or door. Cutting a window well into a poured foundation is routine work but not free.
- Water and moisture. Weeping tile, sump and waterproofing issues must be solved before finishes go in, not after.
- Electrical capacity. A 100-amp service feeding one house may not carry two dwellings. Service upgrades are excluded from ADU base pricing across the industry, including ours.
Is a garage conversion worth it compared to a detached unit?
A garage conversion makes sense when the garage is already detached, structurally sound and sited where a new build would go anyway — it buys you a separate-entrance, standalone-feeling unit for $80,000–$160,000 instead of $148,000+.
It stops making sense in three situations. First, if the slab was poured as an unheated garage floor without insulation or footings sized for a dwelling — that often means demolition and a new slab, at which point you are building detached anyway. Second, if the roof structure won't take insulation depth without dropping headroom. Third, if you actually value the parking, and street parking on your block is already tight. London council has been actively revisiting residential parking rules for new builds, so confirm the current requirements for your zone before you assume the garage is expendable.
Honest math: a marginal garage frequently costs 80–90% of a new detached unit and delivers a smaller, more compromised home. Get a structural assessment before you fall in love with the idea.
Which ADU type rents for the most?
Detached units command the highest rents and the widest tenant pool, because they read as a small house rather than as part of someone else's home. Separate entrance, separate outdoor space, no shared walls or floors, and no footsteps overhead — those features matter to professional tenants, downsizing parents and young couples alike.
Ranked by rental appeal in the London market:
- Detached unit — highest rent, widest tenant pool, cleanest separation
- Garage conversion — near-detached appeal, usually smaller footprint
- Addition — good light and layout, shares a wall with the main house
- Basement conversion — lowest rent, most price-sensitive tenants, sound transfer is the common complaint
Rental rates in London move year to year and vary sharply by neighbourhood and proximity to Western, Fanshawe and bus routes. Pull current comparables for your specific area rather than relying on a city-wide average, and check the City of London's current short-term rental rules if that is part of your plan — those rules have been under review.
Does my lot suit a detached ADU?
A detached ADU needs three things: enough rear or side yard to meet current setback and coverage rules, a clear construction path for equipment and materials, and a workable route for water, sewer and electrical.
Practical screening questions:
- Side-yard access. Can an excavator and a materials trailer reach the backyard? A 1.2 m gap between houses is a real constraint and adds labour cost.
- Distance to services. Trenching 12 m to the main house is a different budget line than trenching 30 m around a pool, mature maple and patio. Utility connections and trenching are excluded from every ADU base price for exactly this reason — they are site-specific.
- Trees and grade. Protected trees, drainage swales and significant slope all shape what fits and what it costs.
- Lot coverage. London has broadened its additional residential unit permissions considerably in recent years, and the city has published standardized design catalogues to speed approvals. Rules on unit counts, height, coverage and bedroom caps continue to be debated at council — verify the current zoning bylaw provisions for your address with the City of London before designing.
Our three pre-designed detached models — The Paul, The Maria and The Henry — are sized to fit typical London backyards. Sizes, layouts and published starting prices for all three are on the ADU models page.
What's included in an ADU price — and what isn't?
Every published Kuzyn Builds starting price covers the complete structure and a finished, move-in-ready interior, including planning and building permits. What sits outside it is site and servicing work — sewer and water connections, electrical service upgrades, excavation and trenching, and municipal connection fees — because those are dictated by your lot, not by the model. The full itemized breakdown is on the ADU models page.
Municipal fees change and London has been actively expanding housing incentives and waiving certain development-related fees to hit federal housing targets. Do not budget from a number you read last year — confirm current fees with the City of London, and ask us to itemize site costs after a site visit rather than guessing.
A simple decision framework
- Tightest budget, want income fastest: basement conversion.
- Have a sound detached garage you don't need for parking: garage conversion.
- Housing a parent or adult child, want connection with separation: addition.
- Want highest rent, best resale, warranty coverage, and you have backyard access: detached unit.
For the full picture on financing, permits and process, read our complete guide to ADUs, and if you're comparing companies, our ADU builders buyer's guide lists the questions worth asking anyone you interview.
Frequently asked questions
Do I need a building permit for a basement or garage ADU in London? Yes. Every additional residential unit in London requires a building permit, including interior and garage conversions, and the work must meet Ontario Building Code requirements for ceiling height, egress, fire separation and ventilation. Kuzyn Builds includes planning and building permit work in our ADU pricing; municipal permit and connection fees are billed separately and change periodically.
Which ADU is fastest to complete? Interior and garage conversions are fastest at 2–4 months of construction, versus 4–8 months for a detached unit. Add permit review time on top of construction for any path. Review durations in London fluctuate with application volume, so ask the City for current turnaround times when you apply.
Is a detached ADU covered by warranty? Yes. Detached ADUs built by Kuzyn Builds carry Tarion warranty coverage, which is Ontario's new-home warranty program. Conversions and additions are not eligible for Tarion because they are not new freestanding dwellings — a meaningful difference if long-term protection matters to you.
Can I build more than one ADU on my London property? Possibly — London has broadened its additional residential unit permissions significantly, and multiple units on a single lot are permitted in many situations subject to lot size, coverage, servicing and bedroom limits. Council continues to revise these provisions, so confirm the current rules for your specific zone with the City of London before planning a two-unit build.
Talk it through with someone who has built both
Kuzyn Builds is a family-owned builder with 80+ completed projects and more than a decade of construction experience in Ontario. We build all four ADU configurations, which means we have no reason to push you toward the expensive one if your basement is the smarter project.
Book a free, no-pressure consultation — we'll walk your lot, flag the site costs that don't show up in a base price, and give you honest numbers for each path. Contact us or call +1-519-694-4776. You can also see our full range of services.