Cost to Extend a House 10 Feet Across Full Width in New West

Extending the back of a house by 10 feet across the full width in New Westminster typically costs $150,000 to $300,000 depending on the width of the house, the number of storeys being extended, and the complexity of the foundation and mechanical work involved. For a standard 30-foot-wide single-storey rear extension, expect to land in the $150,000 to $200,000 range. A 40-foot-wide extension or one that includes basement-level work pushes toward $200,000 to $300,000 or beyond.

This type of full-width rear extension is one of the larger residential addition projects — you are adding 300 to 400+ square feet in a single phase, building a complete new foundation wall across the full width of the house, and tying a new roof structure into the existing roofline along a long connection seam. The scale drives costs higher per square foot than a small bump-out because every system in the house potentially needs modification: the main electrical panel may need upgrading, HVAC ductwork needs extending across a larger area, plumbing lines for kitchens or bathrooms in the extension path need rerouting, and the structural engineer must design the opening where the entire existing back wall is removed or significantly modified.

Here is how costs typically break down for a 30-foot-wide, 10-foot-deep single-storey extension in the New Westminster market:

Foundation: $25,000 to $45,000. You need a new perimeter foundation across the full 30-foot width plus the two 10-foot side returns, with footings designed for New Westminster's soil conditions. Many properties in New Westminster, particularly in the Queensborough and lower Sapperton areas, have challenging soil — alluvial deposits, high water table, or fill material that requires deeper footings or engineered soil improvement. A geotechnical report ($3,000 to $5,000) is often required and may reveal the need for helical piles or compacted gravel pads, adding $10,000 to $20,000 to the foundation cost.

Structural and framing: $30,000 to $50,000. Removing the existing back wall across 30 feet requires a substantial beam (often a steel W-section or multiple LVL layers) to carry the loads above — the existing roof, any second-storey floor loads, and the lateral bracing function the wall was providing. The new walls, floor, and ceiling framing for 300 square feet of addition, plus seismic bracing connections per BC Building Code requirements for Metro Vancouver, account for the bulk of this cost.

Roof extension: $15,000 to $30,000. Extending the roof by 10 feet across the full width means either pushing the existing roof plane outward (ideal but requires removing and rebuilding a significant section of the existing roof) or building a shed roof or cross-gable. The roofing, flashing, soffit, fascia, and gutter work across 30+ feet of new roofline is substantial. In New Westminster's wet climate, proper waterproofing at the roof-to-existing-structure junction is critical.

Exterior envelope: $15,000 to $25,000. New siding on three walls of the extension, matched to the existing house. Windows (a full-width rear extension typically has 3 to 5 windows plus possibly a patio door) run $6,000 to $15,000 depending on size and quality. Insulation to meet BC's Step Code energy efficiency targets adds to the wall assembly cost.

Mechanical systems: $15,000 to $30,000. Extending HVAC to 300 square feet of new space, adding electrical circuits and lighting, and potentially relocating plumbing if the extension affects the existing kitchen or bathroom. If the existing furnace or heat pump does not have capacity for the additional space, replacement adds $8,000 to $15,000.

Interior finishing: $20,000 to $40,000. Drywall, flooring, trim, paint, and any built-ins or kitchen/bathroom fixtures if the extension serves those functions. A straightforward living room or family room extension sits at the lower end; a kitchen extension with cabinetry and countertops pushes to the higher end.

Permits, professional fees, and contingency: $20,000 to $40,000. Architectural drawings, structural engineering, geotechnical report, energy adviser, building permit fees (New Westminster bases these on declared construction value, typically $1,500 to $3,000 for a project of this size), and a 10 to 15 percent contingency for the inevitable surprises when you open up the back wall of an older New Westminster home.

New Westminster's heritage conservation area creates an additional consideration. If your home is in a Heritage Conservation Area (which covers much of the Queens Park, Brow of the Hill, and upper Sapperton neighbourhoods), the rear extension may require a Heritage Alteration Permit in addition to the building permit. This adds review time and may impose design requirements — though rear additions are generally more flexible than front-facing changes in heritage areas.

The rear-yard setback in most New Westminster single-family zones is 6 metres, so verify you have at least 16 metres (the existing house depth plus 10 feet of extension plus 6-metre setback) from front to rear property line before investing in design work.

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