I mapped Ottawa's flood plain. 2,805 addresses are in it.

I ran every Ottawa address against the flood plain overlay

Ottawa's zoning by-law carries an overlay that sits on top of whatever zone your lot is in, and most owners have never heard of it. It is called the Flood Plain Overlay. The City publishes it as a map layer of 891 polygons, and this week I measured all 403,145 points in the City's municipal address file against it.

The answer is 2,805 addresses, 2,317 of them primary addresses rather than unit numbers inside a building. That is about one in every 127 addresses in Ottawa.

I ran it three ways. The 2,805 is the City's own spatial engine on exact overlay geometry. My own pass, on simplified geometry, gave 2,820. The conservation authorities' separate 1-in-100 year flood extent gives 2,816. Fifteen addresses of spread along a boundary, small enough to ignore. Same method I used on Manotick's sewers last week.

The word the by-law uses is prohibited

Section 501 of the new zoning by-law says that despite the underlying zone, or any other provision of the by-law, development is prohibited within any area subject to a Flood Plain Overlay.

It hands back a short list of exceptions, and those exceptions decide what your buyer can do with the house.

An addition is capped at 20 per cent of the building's gross floor area or 20 square metres, whichever is less. Twenty square metres is 215 square feet. On any house bigger than about 1,075 square feet that cap binds, and the size of your lot does not change it. That is a mudroom, not a family room.

An accessory building is held to 50 square metres of floor area or a single storey.

One additional dwelling unit is allowed at or above grade, and only in a house that already existed on June 25, 2008. At or above grade rules out the basement suite.

This is the same 2026 by-law rewrite that renamed most of the city's zones. The overlay came through intact.

It is not only riverfront cottages

Sorted by water, the Rideau takes about 1,420 of them. The Ottawa River 340, the Jock 226, Constance Bay 192, the Carp 50.

Sorted by neighbourhood, the list is not what people expect. Vanier North 374. North Gower and Kars 322. Old Ottawa South 290. Then Richmond 243, Constance Bay 242, Manotick 177, Dunrobin 107, New Edinburgh 91 and Old Ottawa East 82.

Four of the top ten are inner-city neighbourhoods on fully serviced streets. Old Ottawa South alone carries more overlay addresses than Dunrobin and New Edinburgh together, and nobody writes flood risk into a Sunnyside Avenue listing.

By street the heaviest is Bayview Drive in Constance Bay at 202. In spring 2019 the City advised residents of 155 homes on that street to leave and cut power to a stretch of it. Then Mark Avenue 110, River Road 92, Rideau Valley Drive South 79, Borrisokane 52, Greensway 41, Armitage 40, Sunnyside 37.

468 of them sit in an area specific version with wider permissions, and the by-law names those: the Village of Constance Bay, Armitage Avenue, Baskins Beach, and Brewer Park, Linda Thom Park and Windsor Park in Old Ottawa South. Everyone else gets the plain rule.

Your buyer needs a conservation authority before a building permit

Three conservation authorities cover Ottawa. Rideau Valley holds roughly 2,030 overlay addresses, Mississippi Valley 575 and South Nation 210.

Since April 2024 all three work off one provincial regulation, Ontario Regulation 41/24. A permit is required before any development or site alteration in the flood plain, and the RVCA states the order plainly: conservation authority approval comes before the municipal building permit, under the other applicable law provisions of the Ontario Building Code.

Skipping that step is not a slap on the wrist. Individual fines run to $50,000 per offence, up to three months, plus a further $10,000 for every day the offence continues.

So when a buyer stands in your backyard and says the addition goes there, the honest answer is that it goes there if two separate bodies agree and the 20 square metres allows it.

Then the insurance question

The Insurance Bureau of Canada reported in July that about 94 per cent of Canadian homes can now buy overland flood coverage, and 88 per cent of those policies cost $300 a year or less.

The other 6 per cent is the problem. About 850,000 homes still cannot get coverage at a workable price, and IBC says they cluster in high flood risk areas. Two per cent of Canadian homes account for more than half of the country's potential flood losses.

Your buyer's lender wants insurance in place before it funds. If the property is one of the hard ones, that is not a haggling point. It is a financing condition that fails.

The federal government announced Canada's Flood Risk Finder in April, a national address by address lookup. The federal page still reads "once available" and Ontario coverage depends on the province opting in, so the conservation authority map remains the source.

Order the file search before you list, not during a condition

The RVCA sells a property file search. You get a written response covering the hazard mapping on the property, the regulations that apply to it, and any previous approvals, outstanding orders or violations on file.

Two things about that. The last item is what ends deals, and you want to be the one who reads it first. And staff ask for at least two weeks to reply, which does not fit inside a five day condition. Order it while you are settling on a price.

If you are in the overlay you also have a disclosure question to work through with your lawyer, and the line between patent and latent is not where most sellers assume it is.

What September does to this conversation

August was sharp. 1,002 sales, down 18.6 per cent on the year and 24.4 per cent from July, against a ten year median July to August decline of 5.8 per cent. Months of inventory went from 3.5 to 4.5, the highest August reading since 2016. Sales to new listings fell to 47.3 per cent, and more listings left the market without selling.

Where the overlay addresses sit it is worse. Ottawa Rural East ran 6.5 months of inventory on a 40.6 per cent sales to new listings ratio, second softest in the city.

In 2021 a buyer with one weekend to decide took the flood plain on faith. With four and a half months of choice they price it instead, and they price it high, because they have no idea what it costs. A file search you already hold, a straight explanation of the 20 square metre rule and a number that reflects both beats a surprise found in week three.

If you own near the Rideau, the Jock or the Ottawa River and are thinking about listing this fall, send me the address and I will pull the overlay and hazard mapping before you settle on a price. 613-262-6545 or fil@613realtor.ca. Or have a look at what is already competing with you.