How do I calculate bricks for a cavity wall extension?

Calculate each leaf separately. A standard cavity wall needs approximately 120 bricks per square metre total (60 for outer leaf facing bricks, 60 for inner leaf if using bricks, or 10.76 blocks per m² for concrete block inner leaf). Add wall ties at 2.5 per square metre.

Should I match existing bricks on my extension?

Yes, for best visual results. Take a sample brick to merchants for colour matching. New bricks will weather over 2-3 years to blend with existing brickwork. Some manufacturers can match specific brick types if you know the original make.

What Building Regulations apply to extension brickwork?

Part L (thermal performance), Part A (structural), and Part E (sound) all apply. Cavity walls must include appropriate insulation to meet current U-value requirements of 0.28 W/m²K or better for walls.

How thick should cavity walls be for an extension?

Modern Building Regulations typically require 100mm cavity minimum for insulation. Total wall thickness is usually 300mm+ (102.5mm outer brick + 100mm cavity + 100mm block inner leaf). Some designs use wider cavities for enhanced thermal performance.

Do I need Building Control approval for an extension?

Yes, all house extensions require Building Regulations approval regardless of size. You'll need structural calculations, energy assessments, and inspections at foundation, DPC, first floor, and completion stages.

Can I use blocks instead of bricks for the inner leaf?

Yes, and this is standard practice. The inner leaf is typically 100mm concrete blocks (10.76 per m²), which are cheaper and faster to lay. Facing bricks are used on the outer leaf only where visible.

How many wall ties do I need for an extension?

Minimum 2.5 wall ties per square metre in the body of the wall, increasing to 4 per m² around openings and at corners. Use stainless steel ties in coastal areas. For a 20m² wall, that's 50-80 wall ties.

What wastage factor for extension brickwork?

Allow 10% wastage for standard extension work. Increase to 12-15% for complex shapes, multiple openings, or decorative features. This accounts for cuts around windows/doors, breakage, and variations.

How long does it take to build extension walls?

A professional bricklayer lays 400-600 bricks per day. A typical single-storey rear extension (30m² external walls) requires approximately 3,600 facing bricks and takes 6-8 days for the brickwork alone. Weather delays can extend this.

What insulation goes in extension cavity walls?

Full-fill cavity insulation (mineral wool batts or rigid PIR boards) or partial-fill with retained air gap. Insulation must achieve Part L U-value requirements. Common products include Celotex, Kingspan, or Knauf Dritherm.

Back to Brick Calculator

Brick Calculator for House Extensions

Planning a house extension? Calculate the exact bricks needed for cavity walls, including inner and outer leaf calculations. Our calculator helps you estimate materials for single-storey, double-storey, and wrap-around extensions to UK Building Regulations standards.

Cavity Wall Note

House extensions require cavity walls. Calculate outer leaf (facing bricks) and inner leaf (typically blocks) separately, then combine totals.

Bricklaying Materials Calculator

Calculate cement, sand and mortar using standard 4:1 ratio

1. Material Type

2. Wall Length (metres)

3. Wall Height (metres)

4. Waste Factor: 10%

Typical: 10% standard, 5% simple work, 15-20% complex patterns

Tips for House Extensions

Cavity walls are mandatory for habitable extensions under Building Regulations
Match existing brick bond pattern for seamless integration with your home
Order 10% extra to account for cuts, damage, and future repairs
Factor in lintels, DPC, and wall ties separately from brick count

Understanding Extension Cavity Walls

All habitable house extensions in the UK require cavity wall construction to meet Building Regulations.

Typical Cavity Wall Specification

Outer leaf: 102.5mm facing bricks (60 per m²)
Cavity: 100mm with full-fill insulation
Inner leaf: 100mm concrete blocks (10.76 per m²)
Wall ties: Stainless steel, 2.5 per m²

Example: 4m × 3m Single Storey Extension

Perimeter: (4 + 3 + 4) × 2.4m = 26.4m² gross
Less openings: 4.7m² = 21.7m² net
Outer leaf: 21.7 × 60 = 1,302 bricks + 10% = 1,433 bricks
Inner leaf: 21.7 × 10.76 = 234 blocks + 10% = 258 blocks

Common Extension Sizes

Small Rear Extension

3m × 3m single storey

• External walls: ~22m²
• Facing bricks: ~1,450
• Concrete blocks: ~260

Medium Rear Extension

4m × 4m single storey

• External walls: ~29m²
• Facing bricks: ~1,900
• Concrete blocks: ~345

Large Rear Extension

5m × 4m single storey

• External walls: ~31m²
• Facing bricks: ~2,050
• Concrete blocks: ~370

Two Storey Extension

3m × 4m over two floors

• External walls: ~48m²
• Facing bricks: ~3,200
• Concrete blocks: ~570

Frequently Asked Questions

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Related Calculators

Garden Walls Boundary Walls

⚠️ Building Regulations

All house extensions require Building Regulations approval. Contact your local Building Control office before starting work.

Part L Requirements

External walls0.28 W/m²K

Floors0.22 W/m²K

Roofs0.16 W/m²K

Windows1.6 W/m²K

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