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How Much Does It Cost To Build A House Extension

How Much Does It Cost To Build A House Extension

Extending your home is an excellent way to gain extra space without the need to move. However, the cost of building a house extension can initially seem overwhelming. It’s not just about the construction expenses; understanding the various rules and regulations that apply to your project can add complexity and cost.

Firstly, there are many factors that influence the overall cost of an extension. The size, type, and complexity of the extension play significant roles. For instance, a single-storey extension will generally cost less than a double-storey one.

Additionally, the choice of materials and finishes will also impact the total expense. Furthermore, you must consider the fees for planning permission and design, which can vary depending on the scope of your project.

If delving into these details seems daunting, don’t worry. You can always schedule a free 20-minute free professional consultation with one of our experienced home improvement architects. They can help answer your specific questions and provide tailored advice for your project.

In the meantime, let’s explore some typical costs and factors that will give you a clearer picture of how much it will cost to build your house extension.

This way, you can better plan and budget for your exciting home improvement project.

Building House extension Costs

The cost of building a house extension is influenced predominantly by its size, measured in square metres. As a rule of thumb, the larger the extension, the more expensive it will be. Typically, the cost per square metre starts around £1,500 for basic quality.

For better materials and finishes, you should expect to pay approximately £1,900 per square metre. For high-end finishes and custom work, the cost can rise to about £2,200 per square metre.

These prices provide a baseline, but they can fluctuate based on several factors such as design intricacies and material choices. Therefore, understanding these base costs helps in budgeting accurately and setting realistic expectations for your project.

Rear/Side Extension (Single Storey)

For a single-storey rear or side extension, which is often used to create additional living space or a larger kitchen, costs are generally based on the extension’s size. For an average single-storey extension of 20 square metres, the build costs vary significantly depending on the quality of work.

The basic quality option starts from around £30,000, covering essential structural work and minimal finishes. If you choose good quality materials and finishes, the cost rises to approximately £38,000.

For an extension that includes premium materials, high-quality finishes, and potentially more complex designs, you should budget around £44,000.

This pricing typically includes structural work, basic plumbing, and electrical work, but it may not cover additional costs such as high-end fixtures, landscaping, or bespoke features.

Double Rear/Side Extension (Two Storey)

A double-storey rear or side extension is a significant investment that can dramatically increase your home’s space and value.

For such extensions, the costs start from approximately £53,000. This higher cost accounts for the increased complexity of building a second storey, which involves additional structural reinforcements, more extensive scaffolding, and more detailed planning and execution.

The overall expense also reflects the greater amount of materials and labour required. Keep in mind that this estimate may not include costs for high-end finishes, internal fixtures, or extensive landscaping.

Loft Conversion

Loft conversions are a popular way to utilise unused space in your home. They are typically less disruptive than other types of extensions, but costs vary depending on the type of conversion and the existing roof structure.

Room Loft Conversion: This is the simplest and often the most affordable loft conversion, starting from around £15,000. It involves converting the existing loft space into a functional room without major alterations to the roof structure. This type of conversion is ideal if you need an extra bedroom or office and does not require extensive structural changes.

Dormer Loft Extension: Dormer house extension add extra headroom and floor space by extending outwards from the existing roof. These extensions usually start at around £20,000. If you want to include an en suite bathroom, costs increase to approximately £35,000. Dormer extensions are versatile and can significantly improve the usability of your loft space.

‘Raising the Roof’ or Changing the Roof Structure: This more extensive conversion involves altering the roof structure to create more space and headroom. It typically starts from £40,000 and can be quite disruptive. This type of conversion is ideal for creating a large, open-plan loft area or adding multiple rooms.

Hip to Gable Roof Extension: Converting a hip roof to a gable roof starts from approximately £25,000. This modification maximises the loft space by changing the roof’s shape, allowing for a more spacious and usable area. This conversion is beneficial if you have a hip roof and want to significantly increase the loft’s potential.

By understanding the different types of extensions and their associated costs, you can make more informed decisions about your home improvement project. This knowledge will help you plan effectively, manage your budget, and achieve the results you want.

A single storey extension of 20 square metres of basic quality will will cost you a minimum of £44,000 including most fees, but excluding interior fit-out costs.

Interior Fit-Out Cost Guide for House extension

(These costs are in addition to the build cost of the house extension)

  • Kitchen
    Budget from £7,000 to £15,000 – the biggest cost factor is the appliances – the better the appliances, the higher the cost
  • Bathroom
    Budget around £5,000 assuming you can connect to existing pipework
  • Utility Room
    Budget from £1,500 to £11,000 depending on size and quality of appliances
  • Flooring
    Budget from £25 to £100 per square metre
  • Underfloor Heating
    Budget from £10,000 to £15,000
  • New Boiler
    Budget from £2,000 to £3,000 depending if the boiler is being fitted in an existing or new location
  • Wall and Ceiling Finishes
    Budget around £85 per square metre for plaster or dry-lining plus paint
  • Bi-fold or Sliding Doors
    Budget from £1,500 to £2,000 per linear metre

Professional Fees

  • Measured survey – £500 to £1,500
  • Planning and construction drawings – £2,700 to £4,000
  • Architect design fees – three to seven percent of the build cost (e.g. a fee of £900 to £2,100 for a £30,000 extension)
  • Structural engineer – £500 to £1,000
  • Contract management (employing someone to hire the builders/tradesmen) – three to seven percent of the build cost (e.g. a fee of £900 to £2,100 for a £30,000 extension)
  • Project management by a building contractor or architect will be 15 to 20 percent (up to £4,500) on top of the net cost of labour, materials and overheads
  • VAT at 20 percent of the labour, materials and services if applicable

The house extension of 20 square metres at a build cost of £30,000 could potentially attract professional fees from £12,000.

FREE GUIDE: Category: Home Extensions
FREE: Home Extension Cost Guide

Inside this easy to ready free guide, you will learn how much extensions, loft conversions and other renovations to your home are typically priced.

Permission Fees

Additional reports that may be required

  • Tree report (requires an arboriculturist)from £300
  • Ecological ‘Phase 1’ report (required if your project might impact protected species)  – from £600
  • Flood risk/drainage/archaeological/environmental assessment – from £350
    (If problems are found, then this will add additional charges to rectify the issue)
  • Historic building report (if your home is a listed building) – £750 to £1,000 on average for a typical three-bedroom listed building

Other

  • Building Regulations Approval
    You don’t need to get approval yourself if you use someone registered with a competent person scheme. If you are carrying out the building works yourself, or you’re working with someone who’s not registered with a competent person scheme, then budget from £400 – £750
  • Party Wall Agreement
    (Covers shared walls/structures and protects both yourself and your neighbours whilst work is being carried out) You can issue a Party Wall Notice yourself (for no charge) to each of your affected neighbours. If your neighbour agrees to the works in writing, you will not require a Party Wall Agreement.

    If the neighbour refuses to give consent or fails to respond within 14 days, then a surveyor will need to be appointed to arrange a Party Wall Award that will set out the details of the work which will carry a fee of £700 to £1,000 per neighbour

  • Build Over Agreement
    (If you’ve got a shared drain running across the back of your garden and you’re going to build over it or within three meters proximity)
    Budget around £335

All together, it’s worth setting aside at least £2,000 to cover any fees for permissions and sign off

  • Insurance
    If you are carrying out conversion/house extension works you should arrange specialist insurance to cover the new works and the existing structure as most home insurers won’t cover works if contracts exceed £50,000. For expert advise on this and any other insurance exposures, please contact Gary Philip at GPS Insurance Brokers on 020 8207 7385. GPS Insurance

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Total Cost

Single Storey

For a single storey extension of 20 square metres of basic quality, this will give you a minimum spend of £44,000 including most fees, but excluding interior fit-out costs.

Two Storey

For a two storey extension, if the second storey is the same size as the first then the generally-accepted rule is to add 50% extra to the cost of a single storey extension plus an additional 10% for professional fees and VAT.

This gives you a minimum spend of about £87,000 including most fees, but excluding interior fit-out costs.

 

Getting An Accurate Figure

To calculate the cost of your project, Yoop Architects take a measured survey of your property, which captures all the information. Then we draw it all up in 3D and create sketches to show sometimes five or six options if it’s not entirely clear how you want to achieve your brief.

A lot of the time, there are going to be different ways to achieve your desired result and the more you work with the building, the cheaper, easier and faster your build will be. Once you’ve got a favorite scheme or a couple of favorite schemes, we have a phase called ‘Meet the Builders’. That’s when we can introduce you to a couple of builders, who will visit the property and give you a ballpark figure.

Factors Affecting the Cost of a House Extension

Type of Extension

When planning a house extension, the type of extension you choose has a significant impact on the cost. Single-storey extensions are generally more affordable compared to double-storey extensions.

This is because single-storey extensions involve less structural complexity and fewer building regulations. On the other hand, double-storey extensions require additional support and reinforcement, making them more expensive.

In addition, there are specialised extensions such as conservatories, orangeries, and garden rooms. Conservatories are typically the least expensive of these options due to their lighter structures and use of pre-fabricated materials.

Orangeries, which feature more solid walls and a lantern roof, offer a blend of inside and outside space but come at a higher cost. Garden rooms are versatile and can serve as extra living space or an office, but their costs vary depending on the level of insulation and finishing.

Size and Complexity

The size of your extension, measured in square footage or square metres, plays a crucial role in determining the overall cost. Larger extensions naturally cost more due to the increased amount of materials and labour required. Additionally, the layout and design complexity can greatly influence the cost.

A simple rectangular extension is generally cheaper than an extension with an intricate design, such as a multi-level or irregularly shaped structure.

Complex extensions may also require additional structural support and specialised tradespeople, increasing the overall expense. The complexity of design can affect not just the construction costs but also the time needed to complete the project. Therefore, careful planning and design are essential to keep costs under control while achieving your desired outcome.

Materials and Finishes

The choice of materials and finishes significantly impacts the cost of your extension. High-end materials, such as bespoke windows, premium bricks, or natural stone, can dramatically increase costs. Conversely, standard materials like uPVC windows or concrete blocks offer a more budget-friendly option.

Similarly, interior and exterior finishes play a critical role. For instance, high-quality flooring, custom cabinetry, and designer fixtures can add considerable expense to your extension.

On the other hand, opting for standard finishes can help keep costs within a more manageable range. It’s essential to balance your budget with your aesthetic preferences to ensure that you achieve a satisfactory result without overspending.

By considering these factors—type of extension, size and complexity, and materials and finishes—you can make more informed decisions and better manage the costs associated with your house extension.

Cost-Saving Tips

Budget-Friendly Design Choices

When planning a house extension, making budget-friendly design choices can significantly reduce your costs. Opting for a simpler design rather than a complex one can save you a considerable amount.

Simple designs often involve straightforward shapes and minimal structural modifications. These are generally easier and quicker to build, which translates to lower labour and material costs.

For instance, a rectangular extension is typically less expensive than an extension with multiple angles or irregular shapes.

Additionally, avoiding features that require custom work, such as intricate architectural details or high-tech installations, can also keep costs down. Choosing standard designs and materials that are readily available will help you stay within your budget while still achieving a functional and attractive extension.

Moreover, consider using prefabricated components or modular designs. These can often be more affordable than bespoke solutions, as they benefit from economies of scale and streamlined production processes. In summary, keeping your design simple and utilising cost-effective solutions can make a significant difference to your overall expenditure.

Simple Designs vs. Complex Designs

The complexity of your extension design directly affects the cost. Simple designs are generally more cost-effective because they require less time and fewer resources to build.

For example, a single-storey extension with a straightforward layout is usually cheaper than a multi-storey or irregularly shaped extension.

Complex designs often involve additional structural work, more intricate finishes, and potentially higher labour costs. These designs might require specialist tradespeople, detailed planning, and more extensive building permissions.

All these factors contribute to higher overall costs. Additionally, complex designs can lead to longer construction times, which might further increase expenses due to extended labour costs and potential delays.

Therefore, if you aim to save money, opting for a simpler design can be a prudent choice. It’s crucial to balance your aesthetic desires with practical considerations to manage your budget effectively.

In conclusion, simple designs not only streamline the building process but also help keep costs under control while still allowing for a successful and satisfying extension project.

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