Building a Basement in 2025, Complete Guide

How to build a basement so that it's safe, healthy, watertight and pleasant to live in?

Recommendations to build a basement in London

To build a basement is usually quite expensive, especially when excavated below an existing building.

• Be cautious before you choose a company offering a quote that feels too low: that might mean no mitigation against risks was considered when evaluating the price of the basement construction.

• Make sure you hire a company with the right experience and technical competence: check their portfolio and possibly find references.

• Always demand the following:

  • soil test, contamination report are a must

  • construction method statement must be produced beforehand, to describe all the details of how the basement is going to be constructed. It must include H&S and temporary support solutions

  • a structural engineer should be on site to oversee the works, especially during the excavation and the creation of the shell

  • waterproof guarantee must be produced at completion

  • building regulation certificate must be obtained to make sure the construction of the basement was implemented according to regulatory standards

Is building a basement in London worth the risks?

The risk of building a damp and cold basement should not prevent you from an investment that can increase significantly the value of your property and add more space even under urban areas. The right basement construction methods ensure perfectly watertight environments that are as durable as constructions above ground. A healthy environment is also achieved using mechanical ventilation. Even natural light is now easier to get into basements using advanced materials like structural glass.

Still, building a basement needs special professional expertise to avoid issues in the stability, safety and habitability and to minimise the maintenance.

Check our page on basement conversions in London for everything you need to know for your basement project in London

Designing a basement conversion in London

When designing a basement, the thickness of the walls and the area of the steel reinforcement vary heavily according to the load coming from above and to the soil conditions. The depth of the basement also affect the thickness of the walls and the layout of steel reinforcement: the deeper you go the thicker the walls and the steel reinforcement spacing must be. This is to compensate the lateral pressure that increases with the depth.

It is possible to bring natural light to a basement, with structural glass or basement light shafts, that can get natural light underground using a set of mirrors inside a light well.

The quality of the living space in a basement is paramount in a good design.

Basement excavation and the soil in London

The soil and its composition are the first factor to consider. A soil test is necessary for the engineer to design the final structure. Aspects that affect the engineering design are the ground conditions, the soil types and characteristics, the level of the water table and the presence of any contaminants in the soil.

The type of soil defines how it resists compression and load. Those characteristics affect the depth and the type of foundations required. For example, the most common soil type in London is London clay, which is considered adequate to support foundations for a basement. But there are areas where the composition of the soil is different and appropriate solutions must be identified.

Professionals use maps and soil testing to identify the composition and the layering of the soil, so to make sure that they have the right resistance to build a basement on. Lab tests ensure that the characteristics of the soil are adequate to set foundations on.

Other characteristics of the area, like the level of the water table, have an impact on the design of a basement.

Basement excavation: doing the construction right

To build a basement, an engineer needs to design foundations, walls and bases that will make the final structure. But the design must also include measures to support the elements with temporary devices during the construction work. Those devices ensure that the existing building is always correctly supported while the work is carried out. Horizontal support ensures the surrounding soil doesn't collapse and invade the excavated area, while vertical supports guarantee that the over-standing building is correctly supported.

Basement construction methods

Basement construction methods include precise sequences of actions to prevent damages to surrounding properties, above and below ground. When designing the structure, the engineer must consider buildings located very close to the basement site. Temporary supports must be used to prop up the adjoining buildings if necessary and collapse of the soil that supports adjacent buildings must be considered and prevented. Big service infrastructures, like the Underground, and electricity, gas and water pipe layout have an impact as well. Preliminary investigations are required to avoid interfering with existing infrastructure.

How the weather and other external conditions impact a basement project

The weather can have a big impact on the construction of a basement. If you start the excavation in winter, for example, rain and snow might significantly impact the work and cause delays and other types of disruption. For this reason, the months of the year when rain, snow, freeze are more likely should be avoided when planning the construction of a basement. Protections like temporary roofing can be implemented, but they have an impact on the costs and definitely increase the complexity of the site management.

The period of the year can also affect availability of materials, delivery schedules and availability of specialised personnel.

Basement Construction Step by Step Guide:

Step 1: Planning and Permissions

Essential Approvals

• Submit planning application to council if extending beyond permitted development

• Get Building Regulations approval for structural work

• Get Party Wall surveyor if affecting adjacent properties

• Get soil investigation and structural engineer calculations

Step 2: Excavation and Foundation Preparation

Safe Excavation Process

1. Mark out basement area with 600-900mm working space

2. Excavate in stages with proper shoring for depths basement over 1.2m

3. Use CAT scanner to check for underground services

4. Compact formation level and install 150-200mm Type 1 sub-base

5. Lay 50mm sand blinding and DPM

Step 3: Structural Construction

Foundation and Walls

• Pour concrete base slab (C25/30 mix) with A393 mesh reinforcement

• Build walls with concrete, concrete blocks or engineering bricks

• 50mm concrete cover on all reinforcement

• Install continuous insulation (100-150mm PIR boards) for thermal performance especially for basement walls.

Step 4: Waterproofing Systems

Essential Waterproofing

1. Apply structural waterproofing protection and drainage systems (Type A, B or C to BS 8102)

2. Install cavity drain membrane for internal protection

3. Seal all service penetrations with waterproof compounds for water management system.

4. Radon barrier if required by environmental health of the basement structure.

Step 5: Services and Floor Construction

Services Installation

• Electrical circuits to BS 7671 (18th Edition)

• MVHR with heat recovery

• Drainage with sewage ejector pump if below sewer level

• Suspended floor or 150mm concrete slab

Step 6: Access and Internal Finishes

Completion Works

1. Form staircase opening with proper structural support

2. External basement door with weatherproof threshold

3. Sand/cement render (3:1 mix) to walls

4. 65mm floor screed with moisture barrier

5. Moisture resistant plasterboard and skim finish

Final Step 7: Backfilling and External Protection

External Works

• Backfill in 300mm lifts with granular material, compact to 95%

• Land drainage and surface water management

• Protective apron around basement entrance* Render exposed concrete

UK Standards for Building a basement project timescales

Compliance: All work must be to Building Regulations 2010 and British Standards including BS 8102.

Timeframe: 4-8 weeks to complete basement build.

Cost: £1,500-£3,000 per m² fully finished including services.

Professional: Basement specialists like RICS-qualified engineer and CHAS-approved contractors for complex jobs.

Key: Update insurance and warranty for all structural work to protect your investment. Read more articles on construction best practices.

Basement FAQs

Q: Do I need planning permission for my underground extension? A: Yes if you’re extending beyond permitted development rights. Construction Regulations approval is always required for structural work.

Q: What should I budget for a finished underground room? A: £1,500-£3,000 per sqm including all services and finishes. This range is for varying complexity and ground conditions.

Q: Is London clay good for underground construction? A: Generally yes but soil composition varies. Get a professional soil test done for proper structural design.

Q: What qualifications should my contractor have? A: Hire RICS-qualified engineers and CHAS-approved contractors. Check portfolios and references for similar underground projects.

Q: What guarantees should I expect? A: Demand a waterproof guarantee upon completion and Construction Regulations certification. Update your building insurance to reflect increased property value.

Q: Why are some quotes so much lower than others? A: Be wary of low quotes - they often mean no risk assessment or cutting corners on waterproofing and structural support.

Planning your own basement project in London

If you are wondering whether a basement would be the right choice for your property, get in touch to discuss all the aspects with us. Because, why not?