What is the cost difference between a slab and full basement addition in Ottawa?

Choosing between a slab-on-grade foundation and a full basement is one of the biggest cost decisions in an addition. A full basement costs noticeably more because the whole footprint is excavated and full-height walls are built, but it also gives you a second level of space. There is no reliable per-square-foot figure for either in Ottawa; the difference depends on your soil, groundwater and design.

As broad perspective only, the extra cost of a full basement over a slab for a modest addition can run into the tens of thousands of dollars. Ask contractors to price both options with everything else identical so you can see the real gap for your lot.

How each works:

• Slab-on-grade. Perimeter footings go down to the required depth; the interior is compacted granular fill, insulation, a vapour barrier and a concrete slab. Less excavation, less wall and less waterproofing.
• Full basement. The entire footprint is excavated, full-height walls are formed and poured, and the exterior is waterproofed with weeping tile and drainage before backfilling.
• Crawlspace. A middle option that gives access to plumbing and ductwork without a full basement.

On clay or poorly defined soils, the Building Code calls for footings at least 1.2 m deep and not less than the depth of frost penetration, and on sensitive clay footing sizes must be engineered. Deeper, wider excavation for a basement means more soil removal and a greater chance of meeting groundwater, which can add pumping costs. A geotechnical investigation, if your engineer asks for one, tells you what to expect.

The basement case: extra usable space for a recreation room, office or storage. If you are thinking of a separate unit, the City always requires a permit for a secondary unit and the drawings must show the fire separation, so plan for that from the start.

The slab case: lower cost, less disruption and a good fit for a single-storey family room or sunroom, or where groundwater is high.

Energy requirements apply either way. An addition must meet a compliance package or the addition table in the Code's energy supplement, and your designer will specify the slab or wall insulation.

The building permit fee is $12.75 per $1,000 of construction value (minimum $117). You can browse addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare itemized quotes for both foundation types.

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