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How Much Does a Double Storey Extension Cost in the UK?

In the ever-evolving landscape of UK real estate, the quest for space and functionality has led many homeowners to consider the prospect of a double storey extension.

29 August 2023
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Eugene Kim

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This significant decision can provide the much-needed room to grow, adapt to changing needs, and enhance property value. However, within the realm of this ambitious endeavor lies the critical aspect of cost assessment. In this comprehensive article, we will delve deep into the multifaceted factors that exert influence over the cost of a double storey extension in the UK. By gaining a thorough understanding of these intricacies, homeowners can embark on their extension journey equipped with informed insights.

double storey extension

Factors Influencing Cost

1: Size and Scale: The size of the extension stands as one of the foremost determinants of cost. As a general rule, larger extensions necessitate a higher allocation of resources, including materials, labour, and time. The complexity of constructing a double storey extension inherently involves more square footage than a single-storey counterpart, and consequently, the cost scales accordingly.

2: Design Complexity: Architectural vision is another factor that can significantly impact the overall cost. Homeowners with a penchant for distinctive designs, intricate features, or unique layouts should be prepared for the potential rise in expenses. These designs may demand specialised construction techniques, premium materials, and skilled craftsmanship, all of which can elevate the cost.

double storey extension

3: Materials and Finishes: The materials selected for the extension exert substantial influence on the total cost. Opting for high-end materials and luxurious finishes can considerably raise expenses. Conversely, budget-conscious choices in materials can help maintain a more manageable project cost. It is essential to strike a balance between the desired aesthetic outcome and the financial feasibility.

4: Foundation Work: The foundation of any construction project is its literal and figurative groundwork. Depending on factors such as soil type, ground stability, and existing conditions, the cost of foundation work can vary. Extensive excavation, piling, or underpinning requirements can escalate the overall cost of the double storey extension project.

5: Structural Changes: Seamlessly integrating the new extension with the existing structure often demands structural adjustments. Additional supports like steel beams or columns may be imperative to ensure the stability and safety of the extension. However, these structural enhancements introduce an additional cost factor that must be considered.

6: Utilities and Services: The extension’s utility needs, including plumbing, electrical wiring, and heating systems, contribute to the overall expense. Extending these services to the new space or upgrading existing systems involves meticulous planning and financial allocation. Factor in costs for connecting utilities to the extended areas to avoid surprises during construction.

7: Planning and Permissions: Navigating the regulatory landscape is an essential aspect of any construction project. Acquiring planning permissions and adhering to building regulations can encompass administrative fees and costs. It’s important to note that specific circumstances, such as the size and location of the extension, might warrant special permissions, necessitating dedicated budgeting.

8: Labour Costs: Skilled professionals are the backbone of a successful extension project. The costs associated with architects, builders, electricians, plumbers, and other tradespeople are significant components of the total cost. Labour rates can fluctuate based on the project’s complexity and the regional market conditions.

9: Contingency Fund: Prudent financial planning involves the incorporation of a contingency fund. This financial buffer accounts for unforeseen expenses that can arise during construction. The unpredictable nature of construction projects makes a contingency fund a valuable resource to prevent delays and financial strain.

Cost Breakdown
Understanding the allocation of costs within a double storey extension project can provide clarity and foresight. Here’s an approximate breakdown of expenses in the UK context:

  1. Design and Planning: 5% – 10% of the total project cost
  2. Materials: 40% – 50% of the total project cost
  3. Labour: 25% – 35% of the total project cost
  4. Foundation and Structural Work: 10% – 15% of the total project cost
  5. Utilities and Services: 5% – 10% of the total project cost
  6. Contingency: 5% – 10% of the total project cost

Cost Ranges:

As of my last update in September 2021, the cost per square metre for a double storey extension in the UK typically falls within the range of £1,500 to £2,500. However, it’s imperative to note that these figures are subject to variations based on factors like geographical location, prevailing market conditions, and the unique requirements of your specific project.

Double Storey Extension Cost Breakdown

Cost Category Percentage Range
Design and Planning 5% – 10%
Materials 40% – 50%
Labour 25% – 35%
Foundation and Structural Work 10% – 15%
Utilities and Services 5% – 10%
Contingency 5% – 10%


Embarking on a double storey extension project in the UK signifies a substantial investment in your property’s future. The intricate dance of costs and benefits warrants a thorough exploration of the financial landscape. Size, design intricacies, materials, labour, utilities, and regulatory considerations all intersect to shape the overall cost. Collaborating with seasoned architects, builders, and professionals is paramount to obtaining accurate quotes and guiding you through the multifaceted journey. By embracing the nuances that govern cost allocation and meticulous budgeting, homeowners can confidently embark on their extension venture, ultimately creating an expanded and enriched living space that aligns harmoniously with their vision and needs.
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Eugene Kim

Eugene Kim, Founder and Managing Director of Extension Architecture, has led the firm for over 14 years, consistently delivering quality solutions. His dedication has been key to the company's growth and success.

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