How much does a house extension cost?

How much does a house extension cost?

When considering a house extension, the cost can vary depending on factors such as the type of extension.   material finishes, project location, access to the site, market conditions, decision-making process, and the amount of information provided. While there is no exact science to determine the cost, it’s crucial to plan and manage the project effectively to stay within your budget and achieve your desired outcome.
During the initial meeting with your architect or designer, they may ask about your budget. This information is kept confidential and helps establish if your budget aligns with the design brief. It’s important to assess not only the affordability of the extension but also its potential impact on the overall property value. If the build costs, when combined with the purchase price, exceed the market value of similar properties in the area, it could result in negative equity.

house extension cost - carter designs - architect Harrogate

Several factors influence the cost of a house extension:

Architectural services - Alwoodley house extension

Type of Extension:

Standard extensions with pitched or flat roofs and common materials like brick, render, or timber cladding tend to follow the normal cost trend. Irregularly shaped extensions or those using unique materials may require additional expenses due to the need for specialist subcontractors or extra time for completion.

Material Finishes:

The choice of material finishes can significantly impact costs. Opting for high-end finishes can quickly consume your budget, but alternatives that deliver a similar high-end appearance at a lower cost are available. Keeping an eye out for sales and deals on materials can also help reduce expenses.

Project Location:

While project location ideally should not affect costs, it has been observed that more expensive areas tend to have higher build costs. Affluent neighborhoods often attract higher-priced projects, leading to a potential increase of 30-50% in costs compared to less privileged areas. Analyzing tender returns and having a good understanding of build costs can help identify any overcharging.

Access to the Project:

The builder needs sufficient access to safely transport materials and equipment to the site. If access is limited, such as in mid-terrace properties without separate rear access, additional time and labor costs may be incurred. Foundations may need to be dug manually, and certain materials could require careful handling or alternative entry points. Clearly defining and agreeing on these costs upfront, along with well-prepared drawings and a contract, help ensure smooth progress.

Architectural services in harrogate & london
carter designs & architechture harrogate

Market Conditions:

In a busy market, builders can be selective about the projects they undertake. Waiting periods of up to 18 months for a recommended builder can occur, emphasizing the need for early project planning. Builders may provide higher estimates to secure projects, even if they are not genuinely interested in taking them on.

Decision Making:

Taking the time to make decisions during the planning phase rather than on-site can prevent delays and additional costs. Changing room proportions or reconsidering electrical positions after construction has started may result in extra labor and material charges. Clear visualization aids provided by architects, such as detailed plans, mood images, and 3D renderings, can assist in decision making.

Information Provided:

Providing comprehensive detailed information to the builder is crucial. Hiring an architectural professional who can develop the entire design package, rather than just providing designs, ensures that the concept can be accurately executed on-site. Basic planning drawings may leave gaps that require additional decisions on-site, potentially leading to delays and budget overruns. Agreeing on costs beforehand encourages competitive quotes and avoids unexpected expenses during the project.

new build house - Architecture and Design

So how much does a house extension cost?

It’s important to note that there is no fixed formula for the cost of a house extension due to these various factors. As a general guideline, most clients are advised to work with build costs of around ¬£1950/m2 (excluding VAT) for a basic finish. However, it’s unrealistic to expect a ¬£30,000 budget to cover a 50m2 house extension. It’s essential to consider that this estimate does not include professional fees, planning application costs, or any additional expenses specific to your project.

Our Free Extension Cost Calculator estimates extension costs to help you plan. Here’s how it works:

Step 1: Size and Finish

Enter your extension’s size (m²) and pick a finish level: Standard, Medium, or High.

Step 2: Calculate

Click “Calculate” for an estimated breakdown:

  • Build Costs: Core construction expense, excluding VAT.
  • Professional Fees: 7% – 12% of Build Costs.
  • Permissions and Regulations: Factor in permits and rules.
  • Contingency: 15% of Build Costs.
  • Total Costs: Range of all factors combined.


Important: VAT Not Included

Remember, VAT isn’t included. Factor it into your budget separately.


Accurate Estimates

This calculator is purely a guide and is based on prices we have seen across the country over the last 12 months.  The best way to obtain precise prices is to consult an architect or architectural technician who can provide detailed drawings that allow a builder to price from.  Your project is unique, and professionals tailor costs to your needs.


Ready to explore costs for your dream extension? Fill in the details and get started!

Extension Cost Calculator

Extension Cost Calculator

6 Tips for a more accurate estimate for your house extension,:

1. Define your requirements:

Determine the purpose of your extension and outline your specific needs. This could include adding extra bedrooms, expanding the kitchen, creating a home office, or enhancing living space. Clearly defining your goals will help you communicate your needs to professionals involved in the project.

2. Consult with professionals:

Engage with an architect, designer, or a reputable building contractor experienced in house extensions. They can guide you through the process, offer expert advice, and provide an estimate based on your requirements. Ensure they understand your budget constraints to avoid any unrealistic expectations.

3. Obtain detailed quotes:

Request detailed quotes from multiple builders or contractors. These quotes should include the cost of materials, labor, and any additional services or features specific to your project. It’s crucial to compare quotes to ensure you’re getting a fair price and to identify any variations in the services being offered.

4. Consider additional costs:

Keep in mind that there may be additional costs beyond the build itself. These can include planning application fees, building regulations approval fees, party wall agreements, professional fees (architect, structural engineer), and potential costs for services like plumbing or electrical work. Factoring in these expenses from the beginning will help you avoid surprises later on.

5. Allow for contingencies:

It’s wise to allocate a contingency budget of around 10-15% of the overall project cost. This will account for unforeseen issues or changes that may arise during construction. Having a contingency fund will give you peace of mind and ensure you’re prepared for unexpected expenses.

6. Balance cost and quality:

While it’s important to work within your budget, be mindful of the quality of materials and workmanship. Opting for the cheapest options may result in lower-quality construction, which could lead to issues in the long run. Find a balance between cost and quality to ensure your extension is durable, functional, and adds value to your home.

Remember, house extension costs can vary significantly depending on your location, the complexity of the design, and your choice of materials. The best way to get an accurate estimate is to consult with professionals who can assess your specific requirements and provide tailored costings for your project.  Contact us for a free no obligation quotation today!

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