Basement Construction Guide for London Homeowners
A well-planned basement can unlock valuable extra space for living, working, or leisure. It can also transform how the social spaces within the property are arranged and used.
However, basement conversions are not for the faint of heart. This type of project is complex, highly costly, and requires careful coordination with planning, architectural, engineering, party wall, and construction experts.
Understanding Basement Construction
Building a basement involves more than just digging a hole beneath your home. The process typically starts with underpinning your current foundations, including any party walls, thereby ensuring structural stability to the existing home or structure.
Excavation follows, removing soil beneath the property, then laying a reinforced subterranean concrete box and floor slab.
Waterproofing and tanking are then completed to prevent damp ingress or flooding.
Once the shell or box is complete, the interior rooms and finishes are installed, creating the final finish and all-important usable space.
On the surface, these steps may sound straightforward; however, basement construction is a major engineering undertaking. It demands specialised knowledge, meticulous planning, and professional oversight throughout.
It’s also worth noting that this form of construction is one of the riskiest types of residential work that can be undertaken, especially if it is being completed in close proximity to neighbouring properties. Even the most meticulous and robust designs can result in damage to both the property undergoing work and neighbouring properties.
The Basement Construction Process
Most basement projects progress through a series of key stages:
Step 1: Survey and Design
A thorough site survey is completed, assessing soil conditions and structural requirements. Architects and engineers then create bespoke designs to fit the property’s unique characteristics and the homeowner’s needs.
Step 2: Planning Permission
These designs are submitted to the local planning authority, which assesses the proposal against local regulations and policies. This process ensures developments are safe, sustainable, and in keeping with the surrounding area. Approval times can vary from weeks to months depending on the project.
Step 3: Site Investigation & Final Engineer’s Design
Trial pits are dug near party walls to assess the depth and profile of the footing. The information is then provided to the engineer to finalise their design. Most owners delay this step until planning permission is granted to avoid disruption or design issues arising from the planning process.
Step 4: Party Wall Notices
Once the final design is ready, a party wall surveyor will serve party wall notices on adjoining owners affected by the works.
These are required by law at least 1–2 months before work commences.
After receiving the notice, adjoining owners have three response options:
- Consent, allowing the work to proceed without further steps (this is incredibly rare on basement conversions)
- Dissent and appoint a surveyor to represent their interests (this is the most likely outcome)
- Dissent and appoint an agreed surveyor (one surveyor acts for both owners)
Regardless of the adjoining owner’s response, the building owner is responsible for the surveyor costs.
In the event of dissent, one or two surveyors will:
- Review architectural plans, structural plans and construction method statements
- Conduct a schedule of condition to record the current state of the neighbouring property
- Agree a party wall award detailing the scope of work, protections, access arrangements, and procedures for handling any damage
Given the complexity of the works and the protections afforded to adjoining owners by the Party Wall etc. Act 1996, this step is unlikely to be quick. The following principles usually come into play:
An Advising Engineer
The adjoining owner’s surveyor can request structural input from an advising engineer. Without redesigning the project, their role is to review the proposed works to ensure minimal risk to the adjoining property. You can hear more about this on our video.
Security for Expenses
Under Section 12(1) of the Act, adjoining owners can request security for expenses before construction begins. This ensures funds are available to cover potential costs such as property repairs, temporary accommodation, or professional fees if the building owner abandons the project or causes damage. You can hear more about this on our video.
Movement Monitoring
Movement monitoring is essential for high-risk structural works and involves installing monitoring tags on adjoining properties before construction begins. These tags track any shifts during and after the project to detect early movement, allowing timely action to minimise risks and potential damage. Results, often presented as graphs, are regularly shared with party wall surveyors, typically weekly or fortnightly, ensuring the property’s stability.
Step 5: Excavation and Underpinning
Specialist contractors carefully dig beneath and around the property while supporting existing foundations to prevent settlement or damage. This is a slow and careful process. Homeowners should prepare for a construction period that can last several months, often accompanied by noise, vibration, and disruption. Clear communication with neighbours is therefore vital.
Step 6: Building the Basement Shell/Box
Reinforced concrete walls and floors are cast, incorporating waterproof membranes to safeguard against moisture ingress.
Step 7: Interior Finishing
The space is fitted with walls, flooring, lighting, heating, and ventilation systems, transforming it into a liveable area.
Step 8: Inspections and Certification
Throughout construction, regular inspections ensure compliance with building regulations. Upon completion, a final certification confirms the project’s safety and quality.
Step 9: Post-work Schedule of Condition Reports
Once the work is completed, the surveyor(s) inspect the neighbouring property. If damage is identified, the building owner is responsible for repairs or compensation.
What will it cost?
Basement projects vary widely in cost depending on size, complexity, and finishes. Given the complexity of basement work, these indicative prices are accurate at the date of this article:
Typical price ranges for construction and fit-out are likely to be:
Small Basement circa 40m²:
Small mews house, ground floor flat, or mid-terrace house
Likely to be in the region of £325,000 + VAT.
Medium Basement Conversion circa 60-80m²:
Larger mews house, ground floor flat, or mid-terrace house
Likely to be in the region of £400,000 + VAT.
Large Basement Conversion circa 80-100m²:
Larger house, ground floor flat, mid-terrace, semi-detached, or detached house
Likely to be in the region of £450,000 + VAT.
In our experience, houses that share party walls or are built in close proximity to other houses or structures tend to incur higher basement costs due to increased complexity.
Our Take
Basement extensions are an excellent way to increase living space in tight London homes. However, they require thorough planning, professional expertise, and adherence to legal and safety standards. The costs of this form of construction are high, as is the risk. Therefore, be sure to do the maths and speak with estate agents and valuers to assess whether this form of construction will offer a break-even or increase in property value post-work.
Be sure to employ an architect, engineer, party wall surveyor, and contractor with specific experience in basement construction.
Over the years, we have acted as party wall surveyors on numerous basement conversion projects. Our experience, expertise, and thorough understanding of the necessary considerations ensure that each matter is managed efficiently, saving both time and costs.
Team Qualifications
Our team of surveyors are not only highly experienced but importantly they are also qualified.
We’re proud to confirm our surveyors hold membership status and accreditation to some of the world’s leading professional governing bodies including; the Royal Institute of Chartered Surveyors (RICS), the Chartered Institute of Arbitrators (CIArb), The Chartered Association of Building Engineers (CABE) and the Pyramus and Thisbe Club (P&T).
