House Extensions in Westminster

At Bigness Group, we design and build premium house extensions in Westminster, helping homeowners create refined, functional spaces that enhance both living quality and property value.
From Georgian and Victorian townhouses in Marylebone and Pimlico to luxury apartments in Mayfair and Belgravia, we deliver bespoke extensions that combine architectural integrity with contemporary design.

We handle everything — design, planning, structural engineering, and construction — ensuring a smooth process and a beautiful, regulation-compliant result.

Design and Build Services

Our Westminster design-and-build team provides a fully integrated service from concept to completion:

• Detailed survey and architectural design tailored to Westminster’s planning rules

• Planning and listed-building applications through Westminster City Council

• Structural calculations and Building Control compliance

• Interior layout, lighting, and kitchen design integration

• Dedicated project management and milestone tracking

With Bigness Group, you get a single, accountable team coordinating every detail for flawless execution.

Types of Extensions We Build

We construct all major types of residential extensions across Westminster, including:

• Glass Box Extensions – Minimalist designs with a focus on light and transparency.
• Modern House Extensions – Clean, minimalist structures with premium glazing.
• Single-Storey Extensions – Perfect for open-plan kitchens or family spaces.
• Double-Storey Extensions – Add significant space for bedrooms or offices.
• Side-Return Extensions – Ideal for period terraced homes in Marylebone or Paddington.
• Rear Extensions – Elegant garden-facing additions with natural light and glazing.
• Wrap-Around Extensions – Combine side and rear builds for seamless layouts.
• Conservatories & Orangeries – Classic glazed structures for timeless appeal.
• Loft & Garage Conversions with Extensions – Turn unused spaces into practical, comfortable rooms.
• Basement Extensions – Increase living area while preserving outdoor space.

Construction Standards

Every Bigness project in Westminster adheres to our six-phase quality process:

1. Groundworks & Foundations – Engineered for tight urban plots and heritage structures.

2. Structural Steelwork – RSJs and beams installed for stability and open layouts.

3. Walls & Roofs – Brick match, stone cladding, or render with premium insulation.

4. Windows & Glazing – High-performance systems meeting acoustic and energy standards.

5. Interior Finishes – Full electrical, plumbing, plastering, and bespoke joinery.

6. Final Certification & Warranty – Building Control approval and long-term guarantee.

Local Knowledge

We have deep experience with Westminster Council’s planning and heritage frameworks, including Grade II-listed properties and conservation areas in Mayfair, St John’s Wood, and Bayswater.
Our designs respect London’s historic architecture while introducing modern comfort, efficiency, and value.

Costs and Financing

Every extension is unique, but here’s a general guide:

💡 Factors include size, materials, access, and specification level.

We provide transparent quotes with detailed breakdowns and stage payments.
Financing options may be available through our partner lenders for eligible clients — ask our team for guidance on home improvement financing.