How Long Does a House Extension Take?

David Hepburn
February 9, 2025

Introduction

A house extension is one of the most rewarding ways to increase your living space and add value to your property. But the big question for many homeowners is: “How long does a house extension take from start to finish?” In this post, we’ll break down the typical timeline, covering each major stage of the process—from planning and design to construction and finishing touches. We’ll also provide external resources to help you plan your project efficiently and in compliance with UK regulations.

1. Initial Consultation & Design (2–6 Weeks)

Research and Idea Generation

Consult an Architect: Start by discussing your vision, budget, and functional requirements with a professional. An architect can help you understand what’s feasible given your property layout and local building regulations.
Define Your Goals: Are you looking for a larger kitchen, additional bedroom, or a home office? Defining the purpose of your extension will guide the entire design phase.

Key Tip:
For inspiration and guidelines on best practices, refer to resources like the Royal Institute of British Architects (RIBA). They offer helpful insights into appointing an architect and scoping out your project goals.

Concept Drawings and Budget Estimates

Conceptual Designs: Your architect will produce initial sketches to explore layout options and aesthetics.
Preliminary Budget: An early cost estimate ensures your vision aligns with financial realities.

2. Planning Permission & Building Regulations (8–12 Weeks)

Planning Permission Requirements

Permitted Development (PD): Some extensions may fall under PD rights, meaning you do not need formal planning permission if your design meets certain criteria (size, height, location).
Full Planning Permission: If your project exceeds PD limits or you live in a conservation area, you’ll need to submit a formal application to your local authority.

External Resource:
Check the Planning Portal for up-to-date guidelines on Permitted Development and application procedures.

Building Regulations Approval

Structural Safety & Insulation: Regardless of planning permission, you’ll typically need Building Regulations approval to ensure health and safety standards.
Submission & Inspection: You or your architect will submit detailed plans, and building control officers may conduct on-site inspections at different stages.

Timeframe Considerations:

Planning Decision: It usually takes 8–10 weeks for local authorities to respond to a standard application.
Building Control Approval: Can run concurrently with planning, but any required revisions can extend your overall timeline.

3. Detailed Design & Contractor Selection (2–6 Weeks)

Final Drawings and Specifications

Technical Drawings: Once you have planning and building regulation approvals, your architect will create detailed technical plans.
Material Selection: This includes choosing windows, doors, roofing materials, and interior finishes—factors that can affect both cost and construction time.

Contractor Tender & Selection

Gather Quotes: Obtain multiple quotes to compare costs and gauge expertise. Reputable directories like Which? Trusted Traders can help you find vetted builders.
Contract Negotiations: Finalize terms, timelines, and payment schedules with your chosen contractor.

4. Construction Phase (8–20 Weeks)

The construction timeline varies based on the complexity and scale of your extension:

Site Preparation (1–2 Weeks)
- Setting up access routes, site facilities, and ensuring safety measures are in place.
Foundations & Structural Work (2–6 Weeks)
- Excavation, laying foundations, constructing walls, and installing floors.
- Weather Delays: Keep in mind, poor weather can slow down concrete curing or excavation work.
Shell & Roofing (2–8 Weeks)
- Erecting the main structure, adding roofs, and installing windows/doors.
- Once the building envelope is sealed, you’re largely protected from the elements.
First Fix & Second Fix (3–8 Weeks)
- First Fix: Installation of internal wiring, plumbing, and heating systems.
- Second Fix: Fitting items like radiators, light fixtures, and kitchen/bathroom units after plastering is done.
Finishing Touches (1–4 Weeks)
- Decorating, installing flooring, and final inspections.
- This stage can vary widely depending on your chosen fixtures, finishes, and any custom features.

Overall Construction Duration:

Small Extension (Single-Storey): 8–12 weeks
Large Extension (Multi-Storey or Complex Design): 12–20+ weeks

5. Final Checks & Sign-Off (1–2 Weeks)

Building Control Sign-Off

After construction, a building control officer may perform a final inspection to ensure your extension meets all regulations. You’ll receive a completion certificate as proof of compliance.

Snagging & Adjustments

Even well-managed projects often require small fixes, known as snagging. This might include touching up paintwork, adjusting doors, or ensuring fixtures function correctly.

Factors That Can Affect Your Timeline

Planning Delays: Revisions to the design, neighbour objections, or complex local rules can add weeks (or even months) to the process.
Weather Conditions: Heavy rain, snow, or freezing temperatures can delay excavation and structural work.
Material Availability: Global supply chain issues can affect lead times for certain products, especially bespoke items.
Contractor Availability: Good builders are in high demand. Booking them well in advance ensures you don’t face scheduling conflicts.

How Hepburn Architects Can Help

At Hepburn Architects, we’ve guided countless homeowners through the extension process—from initial concepts to final sign-off. Our approach ensures:

Accurate Design and Planning: We handle regulatory compliance to minimize delays.
Realistic Timelines: We set clear milestones so you know what to expect at each stage.
Quality Assurance: We collaborate with trusted builders to ensure high-quality work that stands the test of time.

Conclusion

The journey from initial concept to completed house extension can span anywhere from a few months to nearly half a year, depending on size, complexity, and unforeseen variables. However, with careful planning, professional guidance, and an understanding of each construction stage, you can streamline the process and achieve a successful, value-adding extension.

If you’re ready to start planning your extension—or if you’d just like more tailored advice—get in touch with Hepburn Architects. We’re here to help you build the home of your dreams.