Can you build a basement under a house in the UK?

Adding a basement to your home can bring many benefits with the additional space; it can be used for extra storage, to create a new bedroom or an office space, and it can easily be built with its independent access, so to add exceptional value to an existing house.

The short answer is yes, you can build a basement under your house in the UK, but there are important considerations and regulations to keep in mind.

But is it legal to build a house with a basement and what is the planning permission process?

To answer all your questions, we’ve put together a complete guide on building a house with a basement in the UK.

Basement Construction

Building a basement under an existing house is a highly specialized process that requires careful planning and expertise. Creating a safe and functional underground space is more than just digging; it’s about understanding the soil type, the condition of the existing foundations and adhering to local building regulations. Before any work begins a full assessment of the existing property is essential to ensure the structural integrity of the house is never compromised during the basement extension process.

A well done basement under an existing property can unlock extra space, change how you use your home and increase property value. Whether you want to add a new living area, storage or even a home office the process must be managed from start to finish. Every stage – from design and engineering to excavation and fit-out – should be overseen by experienced professionals who understand the complexities of basement construction. By prioritising safety, compliance and quality you can create a basement that meets your needs and enhances the long term value and functionality of your home.

Benefits of Basement Extensions

Basement extensions offer many benefits for homeowners who want more room without altering the external footprint of their property. By extending the living space under your existing house you can create basement spaces tailored to your lifestyle – whether that’s a spacious family room, a private gym, a home cinema or simply more storage. One of the many benefits of a basement extension is the opportunity to introduce natural light into what would otherwise be an underground space. Through innovative construction methods such as light wells, larger windows or even garden basement extensions you can make your new basement feel bright and welcoming.

Beyond the immediate extra space a basement extension can also improve the energy efficiency of your home. The earth around you provides natural insulation, helping to regulate temperatures and potentially reduce heating costs. A well designed basement can also increase your property value, making it a smart investment for the future. With the right planning and construction a basement extension will transform your home, giving you flexible high quality living space that adapts to your changing needs.

Is it legal to build a house with a basement in the UK?

Whether you’re digging an underground living space under your existing home or have one planned in your new property design, building a basement is usually legal.

In most cases, you will have to get planning permission from your local authority. The local planning authority is responsible for reviewing and approving planning applications for basement construction. In the year 2017 there was a 183% increase in homeowners submitting planning applications to dig under their own houses. In fact, getting planning permission for a cellar is straightforward, because basements have a very low impact on the exterior of an existing building and in general in a neighbourhood. Check the official UK government planning portal.

For most basement projects, a planning application must be submitted, and the process is managed by the local planning authority.

Be aware that there may be additional restrictions in certain areas, such as conservation zones or locations with special planning policies, which could affect the feasibility of basement construction.

How Much Does It Cost to Build a Basement Under an Existing House in the UK?

Basement Construction Costs

Creating underground space beneath your current property in the UK costs between £3000-£5,000 per sqm making it one of the most cost effective ways to add extra living space to your property. For a standard 40 sqm basement, you can expect to pay between £120,000-£200,000 although costs can vary greatly depending on several factors.

Costs breakdown:

• Basic excavation and structure: £1500 - 2500£ per sqm
• Waterproofing and drainage: £250 - 500£ per sqm
• Internal finishing and utilities (including interior finishes such as insulation, electrical, and plumbing): £1000 - 1800£ per sqm
• Professional fees and permits: 10-15% of total project cost

Core contractors are responsible for constructing the main structural shell of the basement, while waterproofing contractors handle moisture protection to prevent water intrusion. Basement fit out contractors then complete the interior, including flooring, lighting, and decor, to finish the basement refurbishment.

Cost to build a basement under an existing house in the UK varies greatly depending on where you are in the country, London and South East projects are 20-30% more than the national average. Northern regions are generally cheaper to build a basement due to lower labour and material costs.

Factors Affecting Cellar Creation Costs

Several key factors influence how much it costs to build a basement under an existing house in the UK:

Soil Conditions and Ground Type

• Clay soil requires more extensive waterproofing (additional £150-£300 per sqm)
• Rocky ground increases excavation costs by 40-60%
• High water table areas need advanced drainage systems
• Certain soil types or structural requirements, especially in older properties, may necessitate deep foundations to ensure stability and safety

Basement Depth and Size

• Standard 2.4m height basements are most cost-effective
• Each additional 0.5m depth adds approximately £400 - £700 per sqm
• Under garden basement projects are typically 15-20% more expensive due to access challenges
• Garden basement extensions and garden basements can also be more expensive due to planning considerations and the need for careful site access

Access and Site Conditions

• Restricted access sites can increase costs by 25-40%
• Need for crane access adds £500 - £1000 per day
• Terraced properties require party wall agreements, adding £2,000-£5,000

Underpinning Requirements

• Existing house underpinning costs £800 - £1200 per linear metre
• Complex foundation work can double excavation costs
• Victorian properties often require specialist techniques
• Older properties may require specialist underpinning methods and deep foundations to ensure long-term stability

Now that we’ve covered the costs, let’s examine the construction process in detail.

Basement Construction Process Step-by-Step

Earlier we released a guide on how to Building a basement and here let’s take a detailed look at how to do this under an existing house.

Before starting any construction work, it is essential to conduct a geotechnical survey to assess the soil type and inform the design and safety measures for the project.

Construction work for a basement involves complex excavation and underpinning to create usable basement space.

Excavation and Underpinning Process

Adding a cellar under an existing house requires a systematic approach to ensure structural integrity throughout the construction process.

Phase 1: Site Preparation and Survey

1. Detailed ground investigation using boreholes and soil analysis
2. Utility location and marking (gas, electric, water, telecoms)
3. Tree root mapping and protection planning
4. Neighbour notification for party wall procedures
5. Assess whether the property sits on solid concrete, as this affects the feasibility of underpinning and the overall basement extension process.

Phase 2: Sequential Excavation
How to build a basement under an existing house safely requires excavating in sections to maintain house stability:

1. Initial excavation to 1m depth across entire area
2. Install temporary steel supports and props
3. Underpinning existing foundations in 1.2m sections
4. Continue excavation to design depth
5. Install permanent retaining walls as work progresses
   Throughout the underpinning process, it is essential to monitor and maintain structural stability to ensure the safety and integrity of the building.

Sequential underpinning process:

• Never underpin more than 1.2m sections at once
• Allow 24-48 hours between adjacent sections
• Use concrete underpinning or steel piers depending on load requirements
• Continuous monitoring of house movement using crack gauges

The result of this process is the creation of valuable new space beneath the existing house, providing a versatile and functional area.

Waterproofing Requirements

Waterproofing is critical for UK underground development due to high groundwater levels and frequent rainfall. Damp proofing is also an essential part of the waterproofing process, as it helps prevent soil moisture from penetrating basement walls and ensures the longevity and safety of the structure.

Multi-Layer Waterproofing System:

External Waterproofing (Most Effective)

1. Tanking membrane applied to external walls
2. Waterproof render coat
3. Protection boards to prevent damage
4. French drain system around perimeter

Internal Waterproofing (Cavity Drainage)

1. Cavity drainage membrane fixed to internal walls
2. Perimeter drainage channel
3. Sump pump installation with backup system
4. Wall and floor finishes over membrane

Drainage Requirements:

• Basement drainage must connect to main sewer or soakaway
• Sump pump capacity: minimum 150 litres per minute
• Battery backup system mandatory for power outages
• Annual maintenance contracts recommended

Underpinning Techniques for Existing Houses

Underpinning strengthens existing foundations to support the new basement structure. The technique depends on your house type and ground conditions. If you are extending an existing basement to increase space or improve natural light, such as by enlarging lightwells or adding glazing, underpinning techniques may also be required.

Mass Concrete Underpinning (Most Common)

• Suitable for most UK house types
• Excavate under existing foundations in stages
• Pour concrete to extend foundation depth
• Cost-effective for standard residential projects such as double storey extensions

Beam and Base Underpinning

• Used for heavier loads or poor ground conditions
• Steel or concrete beams transfer loads to deeper foundations
• More expensive but suitable for complex projects
• Essential for underground development under period properties

Mini-Pile Underpinning

• Required for very poor ground conditions
• Steel piles driven to stable ground
• Most expensive option but most reliable
• Common for building basement UK projects in clay areas

Critical underpinning requirements:

• Building control inspection at each stage
• Structural engineer certification
• 28-day concrete curing before loading
• Continuous structural monitoring throughout process

Ensuring Fresh Air and Ventilation

When planning a basement extension, ensuring a constant supply of fresh air and effective ventilation is essential for creating a healthy and comfortable living space. Unlike rooms at ground level, basement spaces are more susceptible to issues like dampness, condensation, and poor air quality if not properly ventilated. To address these challenges, modern basement construction incorporates advanced ventilation systems—ranging from mechanical ventilation with heat recovery (MVHR) units to carefully designed passive ventilation solutions. These systems help maintain air circulation, control humidity, and prevent the buildup of mold or stale air.

Proper insulation and waterproofing are also critical, as they protect the basement from flood risk and external moisture, further supporting a safe and pleasant environment. By prioritizing ventilation and regular maintenance, you can ensure that your basement extension remains a comfortable and inviting part of your home for years to come. Investing in professional advice and high-quality construction not only safeguards your living space but also preserves the long-term value and integrity of your property.

Ensuring Fresh Air and Ventilation

When planning a basement extension, ensuring a constant supply of fresh air and effective ventilation is essential for creating a healthy and comfortable living space. Unlike rooms at ground level, basement spaces are more susceptible to issues like dampness, condensation, and poor air quality if not properly ventilated. To address these challenges, modern basement construction incorporates advanced ventilation systems—ranging from mechanical ventilation with heat recovery (MVHR) units to carefully designed passive ventilation solutions. These systems help maintain air circulation, control humidity, and prevent the buildup of mold or stale air.

Proper insulation and waterproofing are also critical, as they protect the basement from flood risk and external moisture, further supporting a safe and pleasant environment. By prioritizing ventilation and regular maintenance, you can ensure that your basement extension remains a comfortable and inviting part of your home for years to come. Investing in professional advice and high-quality construction not only safeguards your living space but also preserves the long-term value and integrity of your property.

UK Building Regulations and Legal Requirements

Do you need planning permission?

Whether you need planning permission for basement building at all depends on where you live, your property, and what exactly you’re looking to build.

If you’re just looking to convert your existing cellar, it is possible that you won’t need any permission, unless the outside appearance of your home is being altered. But, if you’re actually digging into the ground below your home, you most likely will need planning permission – which is relatively easy to get.

One important thing to keep in mind, though, is that usually planning permissions to excavate basements are granted with several conditions; these are requirements from the council on construction methods, process, site management, traffic management, safety on site, that are meant to be discharged once the construction starts and as it progresses.

Party Wall Act Requirements

The Party Wall Act 1996 affects most subterranean projects in the UK, especially in terraced or semi-detached properties.

When Party Wall Notice Required:

• Excavating within 3m of neighbouring foundations
• Excavating within 6m if going deeper than neighbour’s foundations
• Building a basement under an existing house that affects shared walls or boundaries

Party Wall Notice Process:

1. Serve notice 2 months before construction starts
2. Neighbours have 14 days to consent or dissent
3. If disputed, appoint party wall surveyors
4. A party wall agreement must be established before modifying shared walls, and a party wall surveyor can assist if disputes arise to ensure legal compliance.
5. Party wall award details work scope and protections
6. Costs typically £1,500-£3,000 per affected neighbour

Basement-Specific Party Wall Considerations:

• Schedule of condition surveys before work starts
• Structural monitoring during excavation
• Insurance requirements for potential damage
• Access rights for inspections

Building Control Approval Process

Building regulations approval is mandatory for all basement building UK projects, separate from planning permission.

Required Building Control Submissions:

1. Structural calculations for excavation and underpinning
2. Waterproofing and drainage specifications
3. Fire safety and escape route plans
4. Insulation and ventilation proposals
5. Basement construction method statements

Building Control Inspection Stages:

• Foundation excavation inspection before concrete
• Underpinning inspection at each stage
• Waterproofing membrane inspection
• Structural frame and floor inspections
• Final completion inspection

Key Building Regulations for Cellars:

• Part A (Structure): Foundation and underpinning requirements
• Part C (Site Preparation): Waterproofing and drainage
• Part B (Fire Safety): Escape routes and fire resistance
• Part F (Ventilation): Mechanical ventilation requirements
• Part L (Conservation): Insulation standards

Local Council Variations

Planning permission for basement requirements vary significantly between UK local authorities, with some councils having specific underground policies.

Westminster City Council (Strictest UK Policy)

• Maximum 50% of garden area for basement under garden
• Noise and vibration impact assessments required
• Detailed traffic management plans mandatory
• Additional structural warranties required

Camden Council Requirements

• Building a basement under an existing house limited to single storey depth
• Enhanced party wall protections
• Comprehensive construction management plans
• Regular progress reporting to council

Standard Council Requirements Across UK:

• Underground planning permission applications include:
  • Detailed construction methodology
  • Traffic management during construction
  • Hours of work restrictions (typically 8am-6pm weekdays)
  • Neighbour consultation evidence
  • Environmental impact assessments

Common Planning Conditions:

• Construction Management Plan approval before start
• Party wall awards in place
• Professional indemnity insurance evidence
• Structural warranty provisions
• Noise monitoring during construction

Planning permission regulations for basement construction

To obtain permission to build your own cellar, you’ll need to adhere to both the country-wide regulations as well as your local council’s, which may be stricter depending on where you live.

We recommend you hire an architect to be in charge of your planning permission application and the design of your underground living space, and that you appoint an experienced construction company to do the work. It is also advisable to engage a professional for interior design to optimize the new basement space, ensuring both functionality and timeless aesthetics.

Planning conditions are usually quite strict and you will need a professional construction company to be able to discharge them. Plus, excavating a basement is complex and requires accurate procedures on site, especially around safety and temporary structures.

You also have to check whether you need to serve a party wall notice for your cellar project. For example, a notice is due if you are excavating below a certain distance from your neighbours walls and foundations.

Getting planning permission for a basement is very simple, and it’s definitely worth it considering the benefits of the extra space. Nowadays it’s easy to create an underground living space  that is completely waterproof, well aerated and has still plenty of sources of natural light.

So Can you build a basement under an existing house in the UK?

In summary, building a basement under your existing house in the UK is entirely possible—and can transform your home with valuable extra space. With careful planning, expert construction, and compliance with regulations, you can create a high-quality living area that adds lasting value to your property.