Reinforced concrete basement walls and slabs
- Design of retaining walls, floor slabs and toe beams to resist earth and water pressure.
- Crack control, deflection and reinforcement layouts suitable for site fixing.
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Conversions and extensions • Basement structural design
Basement structural engineer - safe retaining walls, underpinning and slabs
New and converted basements need careful structural design. We design retaining walls, reinforced concrete slabs, underpinning and waterproofing details so your builder knows exactly what to build and Building Control has the numbers they need.
As your basement structural engineer we focus on stability, water pressure, soil heave and neighbour impact, not just the finished room layout.
- Reinforced concrete basement boxes, retaining walls, slabs and underpinning designed as one system.
- Water pressure, ground movement and party wall effects checked in line with UK guidance and Eurocodes.
- Fixed fee basement structural calculations and drawings that your contractor and Building Control can follow.
We support basement conversions, new under house basements and garden basements across the UK, working with your architect, contractor and waterproofing designer from concept to completion.
Basement structural engineer - what we design for your project
We focus on the structure that holds the ground back and keeps the basement dry. These are the main elements we normally design for basement conversions and new basements.
Underpinning and deepened foundations
- Underpinning of existing walls where levels are taken down to form a basement.
- Sequencing notes so the contractor knows which pins to form and backfill first.
Basement retaining walls to gardens and lightwells
- Retaining walls for front lightwells, rear gardens and side access paths.
- Allowance for railings, balustrades, vehicle loads and boundary fences.
Temporary works concept design
- Concept level stability checks for propping and temporary openings.
- Clear notes for the contractor or their temporary works designer to follow.
Existing basement checks
- Review of existing walls, slabs and columns where you plan to change use or layout.
- Strengthening proposals where new openings or higher loads are required.
Waterproofing and drainage coordination
- Structural support for membranes, cavity drain systems and sumps.
- Falls and thickenings in slabs where pumps, channels or drainage are needed.
When you must involve a basement structural engineer
Some basement work is clearly structural. These are the situations where design and calculations are essential for safety and Building Control approval.
Digging below existing foundations
- You are lowering the floor level to create headroom under the existing house.
- Underpinning, new RC walls or a new slab will be needed to carry the loads safely.
Basements near neighbours or party walls
- The basement excavation is close to a shared wall or another building.
- Engineered drawings help the party wall surveyor and reduce disputes and delays.
High water table or poor ground
- Trial pits or boreholes show water in the excavation or very soft ground.
- We check uplift, sliding, bearing and settlement so the basement stays stable and dry.
Basement extensions with large openings
- You want big glazed doors, long spans or column free rooms at basement level.
- We design beams, frames and transfer structures that work with the basement box.
Water pressure, heave and waterproofing in basements
These quick notes cover the issues we discuss most often with clients when planning a new or converted basement.
Water pressure and uplift
We check the slab for uplift from water under the floor and design enough self weight or anchors to resist it.
Where needed we allow for drainage, sumps and pumps so pressure can be relieved.
Ground movement and heave
On clay soils and sites with trees we consider long term heave and shrinkage. Movement joints, slip details
and flexible linings are used so the waterproofing can cope with movement.
Waterproofing strategy
We coordinate with your waterproofing specialist to support the chosen system, whether that is a drained
cavity, tanked wall, or combined approach for habitable basements that need higher protection.
Information we need to design your basement
Good information at the start keeps the design clear and avoids changes on site. This is what we normally ask for when you contact us for a basement structural engineer quote.
Architect drawings and layout
- Plans, sections and elevations showing basement extent, room use and openings.
- Proposed floor levels, external levels and any lightwells or garden retaining walls.
Ground investigation and water level
- Trial pit photos or a site investigation report with soil type and water table.
- Any long term monitoring results if the site is known to be wet or sensitive.
Existing structure and neighbouring buildings
- Construction type of the existing house and foundations where known.
- Distance to neighbouring houses, party walls and boundary structures.
Waterproofing strategy and intended use
- Whether the basement is for storage, plant rooms or habitable use such as bedrooms.
- Any preferred waterproofing system or specialist already appointed.
Basement conversions and extensions - frequently asked questions
Short answers to the questions we hear most often from homeowners, architects and contractors planning a basement project.
Do I need planning permission for a basement conversion?
Many straightforward basement conversions fall under permitted development if you are not changing the front elevation or adding larger lightwells. New basements or extensions often do need planning consent. Your architect should confirm this with the local authority before work starts.
Can I build a new basement under an existing house?
In many cases yes, but it involves significant structural work. Underpinning, temporary works and careful sequencing are required. A basement structural engineer should be involved from the start and a party wall surveyor is usually needed where neighbours are close.
How does water affect basement design?
Water adds permanent pressure to the walls and slabs and can cause uplift under the floor. We check these pressures and design the structure and drainage so that the basement remains stable and dry in the long term.
Who designs the waterproofing system?
On many projects a waterproofing specialist is appointed to design the membrane or cavity drain system. We coordinate with them so the structure supports their details and meets the grade of protection required for the planned use of the basement.
How much does a basement structural engineer cost?
Fees depend on size and complexity. Simple conversions using existing walls cost less than new basements formed under the whole house. After we have seen your drawings and ground information we confirm a fixed fee before any design work starts.
Homeowners who used us for basement structural design
A few short comments from clients who asked us to design their basement retaining walls, slabs and underpinning.
Basement conversion – London
Underpinning and new slab
“Clear staged underpinning details and good communication with the party wall surveyor. The contractor said the drawings were easy to follow and Building Control were happy with the calculations.”
New garden basement – Manchester
RC box and retaining walls
“They explained the options for wall thickness and waterproofing in plain English. We felt confident the structure had been thought through and coordinated with the drainage and pumps.”
Existing basement upgrade – Bristol
Load checks and strengthening
“SECalcs checked the existing walls and beams so we could turn the cellar into a habitable room. The report gave us what Building Control needed without overcomplicating the work.”
Request a basement structural engineer quote
Use this form to outline your basement conversion or new basement. Your enquiry goes straight to an engineer, not a call centre.
Tell us about your basement project
Share a few details so we can review the drawings and ground information and confirm a fixed fee for your basement structural calculations.
This form connects to our central SECalcs email. You will usually hear back from a structural engineer within one working day.
Ready to move your basement project forward?
If you are planning a basement conversion, new basement or garden retaining wall and want clear structural calculations, send us a few details and we will confirm the next steps and a fixed fee.
- Head Office: Corwell Lane, Uxbridge, England, UB8 3DE.
- 07359 267907
- info@structuralengineercalcs.com
- Company No: 12653163