Extending your home is one of the most effective ways to add space, functionality and long-term value to your property and a single storey extension remains the most popular choice for homeowners across St Albans, Harpenden and Hertfordshire.

At ACR Build, we understand that one of the first questions every client asks is:

“How much will my extension actually cost?”

With material prices, labour rates and design standards evolving constantly, 2025 build costs can vary widely depending on your location, specification and complexity.

ACR Build at a Glance

Based in Hertfordshire, ACR Build is a leading contractor specialising in:

• Home extensions & renovations
• New build developments
• Conservation & heritage restorations
• Design & build projects
• Supreme electrical installations

With decades of experience delivering high-quality craftsmanship across St Albans, Harpenden, and the surrounding areas, we combine technical excellence with transparent cost planning to ensure every project is delivered on time and on budget.

1. 2025 Single Storey Extension Cost Overview (UK Averages)

Before planning your home extension, it’s useful to understand the current build cost benchmarks across the UK. Based on 2025 data from trusted industry sources such as BuildPartner, Homebuilding & Renovating, and Studio Colab, the average single-storey extension cost in the UK typically ranges between £1,800 – £4,000 per square metre. The final figure depends on your property’s location, design complexity, materials, and finish quality.

• BuildPartner (2025): Average single-storey extension cost is approximately £2,890/m² (excluding VAT).

• Homebuilding & Renovating (2025): Typical range of £2,200 – £3,300/m² for a standard specification.
• Studio Colab (London focus): Between £2,500 – £4,000/m² for premium finishes.

These figures are based on publicly available 2025 construction data from reputable UK sources and serve as general industry indicators. Actual costs can vary depending on your home’s condition, structural requirements, and regional factors particularly across Hertfordshire, St Albans, and Harpenden, where property standards and planning considerations can influence pricing.

Regional Impact

In Hertfordshire, costs typically sit slightly below London but above national averages due to high-quality housing stock, strong demand and skilled labour rates.
Expect:
£2,400 – £3,200/m² for a well-specified project in St Albans or Harpenden.
£1,900 – £2,500/m² for smaller or more basic builds in outer areas such as Hemel Hempstead or Hatfield

2. Cost Breakdown by Specification

What Drives the Difference?

• Design complexity: L-shaped or wrap-around layouts require more materials and structural work.
• Roof type: Flat roofs are economical; pitched or vaulted ceilings add cost.
• Glazing: Bi-folds, Crittall-style windows or roof lanterns increase budgets.
• Internal finish: Flooring, kitchens, lighting and electrical design make a huge impact on total cost.

3. Typical Total Project Costs (Hertfordshire Examples)

Let’s look at realistic project scenarios ACR Build has costed recently in St Albans and Harpenden:

Note: Heritage or conservation projects typically involve additional planning, specialist materials and approvals, increasing costs by 10–25%.

4. Key Factors Affecting Extension Cost

a) Design & Complexity

Unique shapes, high ceilings or structural steelwork add cost. Simpler footprints give better value per m².

ACR Build’s design & build service helps optimise your layout to achieve design ambition efficiently.

b) Ground Conditions & Site Access

Poor soil, drainage issues or restricted site access (common in St Albans terraces) can add significant cost in excavation and labour

c) Materials & Finishes

Material choice defines both appearance and budget. Locally sourced bricks and tiles can reduce transport cost, while premium finishes raise resale value.

d) Location & Labour

Hertfordshire labour rates are among the UK’s highest outside London. Quality local trades and specialist electricians (like ACR Build’s in-house team) ensure premium workmanship.

e) Professional Fees & Permissions

Architectural design, planning submissions, structural engineering and building control typically add 8–15% to overall project value.

f) VAT

Most residential extensions incur 20% VAT. However, conversions or listed-building works may qualify for reduced rates, ACR Build advises clients individually.

5. Planning, Regulations & Heritage Considerations

Permitted Development

Many single storey rear extensions fall under Permitted Development Rights, avoiding full planning permission, provided they meet limits on size, height and boundary distance.

Full Planning Permission

You’ll need full planning if:

• The extension exceeds PD limits,
• Your home is in a Conservation Area, or
• It’s a listed building.

Building Regulations

All extensions require compliance with Building Regulations covering structure, insulation, ventilation, electrical safety and fire performance

ACR Build handles submissions and inspections, ensuring seamless approval.

Conservation & Heritage Projects

In areas like Harpenden’s West Common or St Albans’ city centre, heritage sensitivity is essential.
ACR Build’s conservation specialists work with local authorities to ensure sympathetic designs that meet modern performance standards without compromising character.

6. Cost-Saving Tips Without Compromising Quality

1. Keep your footprint simple. Rectangular layouts and single-pitch roofs offer best value.
2. Select materials early. Early procurement avoids price volatility.
3. Combine build phases. Adding electrical or renovation work concurrently reduces duplication of labour.
4. Standardise openings. Custom window sizes increase cost; standard modules save thousands.
5. Consider phased interiors. Focus on structure first, upgrade finishes later if budget dictates.
6. Invest in insulation and electrics. Smart electrical design, efficient lighting and heating lower long-term running costs.

7. Return on Investment

A well-designed extension can add 10–20% to your property’s market value in Hertfordshire’s competitive housing market.

Well-executed design & build projects enhance both lifestyle and resale appeal, a strong return for homeowners planning long-term.

8. Why Choose ACR Build

Choosing the right contractor is as important as the design itself.
Here’s why homeowners across St Albans, Harpenden and Hertfordshire trust ACR Build:

• Transparent pricing: Detailed cost breakdowns and no hidden extras.
• Design & Build expertise: In-house design team streamlining planning, drawings and construction.
• Quality craftsmanship: Skilled trades, premium materials and precise finishing.
• Heritage sensitivity: Conservation-approved methods for listed properties.
• Supreme electrical division: Certified electrical installations integrated from the outset.
• Project management excellence: From concept to completion, one dedicated contact ensures clear communication and smooth delivery.

Our goal is not just to build extensions, but to create enduring, energy-efficient spaces that add beauty and value to your home.

9. Frequently Asked Questions

Q1. What’s the average cost per m² in 2025 for a single storey extension in Hertfordshire?
Typically between £2,400 – £3,200/m², depending on specification, site conditions and materials.

Q2. How long does a single storey extension take?
A standard 25–30 m² build usually takes 10–16 weeks, including structural work, electrics and finishing.

Q3. Can ACR Build manage the full project from design to completion?
Yes. Our Design & Build service covers concept design, planning, engineering, construction, and electrical fit-out fully managed under one contract.

Q4. How do I get an accurate quote?
Book a consultation. We’ll assess your property, desired size/spec and site access, then provide a transparent itemised estimate based on 2025 rates

10. Summary & Next Steps

In summary, the cost of a single storey extension in 2025 varies between £2,200 – £3,500 per m² nationally, with Hertfordshire projects typically ranging £2,400 – £3,200 per m².

Your total project could cost:

• £45,000 – £60,000 for a modest, functional extension
• £70,000 – £90,000 for a larger mid-range kitchen-diner
• £100,000 + for a bespoke, premium build

Working with an experienced local contractor like ACR Build ensures your investment delivers both immediate comfort and long-term value, managed by professionals who understand the specific planning, conservation, and build conditions in St Albans, Harpenden and Hertfordshire.

11. Final Thoughts & Expert Advice for 2025 Homeowners

Building a single storey extension is more than an upgrade, it’s a long-term investment in lifestyle, comfort, and property value. As 2025 unfolds, homeowners across Hertfordshire are continuing to prioritise space, efficiency, and sustainability in their builds.

Plan Early, Build Smart

Planning ahead is key. Start with a clear budget, realistic timeline, and professional guidance. Early engagement with a design & build specialist like ACR Build ensures your ideas are technically feasible and cost-effective. From design concept to material selection and regulatory compliance, every decision impacts the final outcome, both in cost and quality.

Energy Efficiency in 2025 Builds

Energy-efficient construction is no longer optional. Homeowners are seeking extensions that not only look beautiful but perform efficiently.
Modern single storey extensions can integrate:

• High-performance insulation and airtightness
• Solar-ready roof structures
• Smart electrical layouts with zoned lighting and heating
• Low-carbon materials and underfloor heating systems

ACR Build’s in-house supreme electrical division ensures every project is future-ready, designed for both comfort and sustainability.

Material Trends & Finishes

In 2025, design trends across Hertfordshire highlight a move toward natural textures, sustainable materials, and seamless indoor-outdoor transitions.
Popular finishes include:

• Aluminium or steel-framed glass doors
• Timber cladding and brick detailing that complements period homes
• Minimalist kitchens with hidden lighting and built-in storage
• Large-format floor tiles or polished micro cement for contemporary appeal

Every finish contributes to the atmosphere and long-term value of your home, which is why ACR Build’s design and construction teams collaborate closely to achieve aesthetic balance and build quality

12. Working With a Premium Contractor: The ACR Build Difference

Choosing the right contractor can make or break your extension project. ACR Build’s reputation across St Albans, Harpenden, and Hertfordshire is built on three key pillars:

1. Precision in Planning

Every successful project begins with accurate planning. Our in-house design and project management teams create detailed cost schedules, structural drawings, and Gantt timelines, ensuring every phase runs efficiently.

2. Craftsmanship in Execution

From brickwork and roofing to fine interior finishes, our skilled tradespeople deliver workmanship that stands the test of time. We work with trusted local suppliers to guarantee consistency, durability, and attention to detail.

3. Communication & Transparency

We believe that premium construction means peace of mind. With ACR Build, clients receive clear communication, transparent pricing, and regular progress updates eliminating uncertainty at every step.

Our goal is simple: to deliver exceptional spaces that combine innovation, functionality, and timeless design.

13. ACR Build’s Local Expertise: St Albans, Harpenden & Hertfordshire

ACR Build’s deep regional experience gives us a unique understanding of the architectural character and planning nuances across Hertfordshire.

• St Albans: Known for its conservation zones and Victorian homes, where sympathetic design and quality materials are essential.
• Harpenden: Characterised by spacious plots and modern family homes, ideal for open-plan kitchen-diner extensions and lifestyle upgrades.
• Wider Hertfordshire: From rural villages to urban developments, our projects adapt to local planning frameworks, sustainability goals, and homeowner needs.

Our heritage and conservation expertise ensures even listed and period properties can benefit from thoughtfully integrated extensions that preserve original charm while introducing modern performance standards

14. Choosing Between Single and Double Storey Extensions

While this guide focuses on single storey extensions, many homeowners also compare them to double storey builds. Here’s a quick comparison for perspective:

Single storey extensions remain the most popular option due to their affordability, flexibility, and minimal disruption, while still offering major improvements in lifestyle and property value.

15. Conclusion

A single storey extension is one of the most effective ways to enhance both the functionality and value of your home, without the stress or cost of moving. With the right design, careful planning, and skilled construction, your property can evolve into a space that perfectly suits your family’s lifestyle and future needs.

At ACR Build, we specialise in crafting single storey home extensions that seamlessly combine heritage character, modern design, and energy-efficient innovation. Our extensive local expertise across Harpenden, St Albans, and Hertfordshire ensures every project is compliant, practical, and beautifully executed from concept to completion.

Whether you want to expand your kitchen, create a new family living area, or design a light-filled open-plan space that connects seamlessly with your garden, our experienced builders and designers can help turn your vision into reality

With ACR Build by your side, you gain more than just additional space- you gain a home that’s built for the future.

Book your consultation today and discover how even a modest extension can transform your property into something extraordinary.

👉 Ready to begin? Contact ACR Build today to start your renovation journey.