West of Yonge Street, Thornhill is Vaughan; east of it, Markham. The two cities draw the permit line in different places for decks and sheds, so check your own city's rule before you start, not your neighbour's.
Thornhill is one of the few places in York Region where two cities share a single neighbourhood name. A house on the west side of Yonge Street and one on the east side can look identical, be listed together and be worked on by the same craftsman, and still fall under different building departments, different zoning by-laws and different fence by-laws. For most small jobs the answer is the same on both sides. For a few, it isn't, and those are the ones that cause trouble when you only find out at resale. This is general information, not legal advice; the cities' own staff have the final word.
First, find out which city you're in
Between Steeles Avenue and Highway 7, Yonge Street is the boundary: Vaughan on the west, Markham on the east. You can't always tell by eye on the side streets, so look at your property tax bill, which names the municipality that issues it. Everything below follows from that one fact.
Decks: the clearest difference
This is where the two cities genuinely disagree. According to Vaughan's building permit pages, an uncovered deck needs no building permit if it is 18.6 m² (about 200 sq ft) or less, no more than 0.6 m (about 2 ft) above grade, and not attached to an existing structure. Any deck or porch attached to a building needs a permit, regardless of size or height.
Markham's published rule is tighter on size: a freestanding rear-yard deck under 10 m² (108 sq ft) needs no permit, while a deck over 10 m², attached to the house, or more than 600 mm (2 ft) above ground does. So a 12 m² freestanding, low deck is permit-free on the Vaughan side of Thornhill and needs a permit on the Markham side.
Neither city treats "no permit" as "no rules". Zoning setbacks to the property lines still apply to every deck in both cities, and a deck that breaks them can be ordered changed even though nobody issued a permit for it. Markham's deck guidance also asks that you protect surface drainage when building.
Sheds and small accessory buildings
Markham: a shed, sauna or similar building with a footprint under 10 m² (108 sq ft) needs no permit; 10 m² or more does. Multiply finished length by width to get the footprint.
Vaughan's pages are less tidy. Its accessory structure guidance says a detached structure needs a building permit at 10 m² or more, if it is attached to another structure, or if it contains plumbing. A separate Building Code Act exemption covers a shed up to 15 m² that is one storey, detached, storage-only and without plumbing, and Vaughan's own wording reflects both numbers. Because the pages disagree with each other, treat 10 m² as the safe planning number and ask the city before building anything bigger. Vaughan also has a grading permit that applies to structures over 10 m², which is separate from a building permit.
Fences
Neither city's published guidance asks for a building permit for an ordinary fence; the permit trigger is a pool enclosure. What differs is the height. Markham's Fence By-law 277-97 allows 1.2 m (4 ft) in a front yard and on the street side of a corner lot, and 1.8 m (6 ft) in interior side and rear yards. Vaughan sets its limits in a table by location: 1.2 m in a front yard within 2.4 m of the street line, and up to 2.0 m for many other residential cases. Vaughan also takes a corner-lot sight triangle seriously, and a fence taller than the table allows needs a formal exemption application. If you are replacing a fence on the same line at the same height, you are usually fine; if you want to go taller, read your own city's table for your own lot first.
Fixture swaps and interior work: mostly the same
The good news is that the everyday jobs line up. Markham's list says no permit is needed to paint, to install new kitchen or bathroom cupboards, to re-shingle, to replace existing taps, tubs, toilets or sinks, or to swap a furnace or water heater for the same type. It does require one to add new plumbing where none existed, to change a furnace's fuel source, or to install a wood stove or fireplace. Vaughan's list likewise exempts replacing existing fixtures. Electrical is separate from both cities: electrical work is regulated through the Electrical Safety Authority, not the municipal building department, so a permit-free job can still need an ESA notification.
Quick answers
Which city is my Thornhill house in?
West of Yonge Street is Vaughan; east of it is Markham. Your property tax bill names your city, so check it before you call anyone about a permit.
Do I need a permit for a deck in Thornhill?
It depends on the side. Vaughan exempts an uncovered, unattached deck up to 18.6 m² and 0.6 m high; Markham's no-permit line is under 10 m². Attached decks need one in Vaughan.
Do I need a permit to replace a toilet or faucet in Thornhill?
No, Markham's list says replacing existing taps, tubs, toilets or sinks needs no permit. Adding new plumbing where none existed does, so a new fixture location is different.
Does a shed in Thornhill need a permit?
Often not when it is small. Markham's line is a footprint of 10 m² (108 sq ft); Vaughan's pages use 10 m² too, but note a separate Building Code exemption up to 15 m². Setbacks apply either way.
Who do I call if I am not sure?
Call your own city, not the other one. Markham's Permit Application Administrator's Desk is 905-475-4870; Vaughan Building Standards is 905-832-8510. Confirm numbers on the city site first. If you want a vetted craftsman to quote the work, you can get a fixed price from Rhodex.
Check once, keep the paper
When a buyer's lawyer asks about a deck or shed, "no permit was required" only helps if it was true on your side of Yonge Street. Write down the date you checked, who you spoke to, and the dimensions you gave them. It takes ten minutes and it is the document that matters later.
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