Strip footings
- Standard footings for rear and side extensions on reasonable ground.
- Widths typically 450–750 mm, depth set by soil, frost and tree influence.
Menu
Extensions and new builds • Footings and foundations
Footings for Extension – safe, practical foundation design for extensions and new builds
Most extensions need new footings to carry the extra load. We check the soil, footing depth, drains and load path so your builder knows exactly what to dig and how deep. Our designs cover single storey extensions, two storey extensions and small new builds.
Most UK projects use simple strip foundations or trench fill, so our footings for extension designs focus on soil, drains and the existing house footings.
- Strip, trench fill, pad, raft and retaining foundations designed as a complete system.
- Foundation depths checked for soil type, trees, drains and existing footings.
- Fixed fee structural calculation packs with clear drawings your builder can follow on site.
We work with homeowners, developers and architects across the UK, providing foundation and footing designs that satisfy Building Control and keep projects moving on site.
Footings for Extension – what we check before designing your foundations
We cover the footing type, depth, width and reinforcement so your builder knows exactly what to build and Building Control has the numbers they need.
Trench fill foundations
- Deep concrete fill that reduces brickwork and speeds up the build.
- Useful where the ground is soft, clay is deep or access is tight.
Pad foundations and beams
- Pads beneath steel columns, portal frames or point loads.
- Ground beams linking pads where walls or openings sit between them.
Raft slabs
- Reinforced slabs with thickened edges for soft or variable ground.
- Helps avoid very deep trenches where trees or fill make footing depth excessive.
Retaining foundations and level changes
- Walls that hold ground where the garden steps up or down near the extension.
- Checks for earth pressure, surcharge from cars and patio slabs.
Foundation checks for new builds
- Strip, trench fill or raft foundations for detached houses and infill plots.
- Designed with your architect so the structure and layout work together.
Foundation depths, footing types and extension regulations
These quick notes summarise the questions we are asked most often about footing depth, types of foundation and what Building Control expect to see.
Typical foundation depths
1.0–1.4 m for many rear extensions on firm ground.
1.5–2.5 m on clay with trees nearby.
600–900 mm where non shrinkable sand or gravel is confirmed.
Deeper or rafts where fill, trees or drains control the design.
1.5–2.5 m on clay with trees nearby.
600–900 mm where non shrinkable sand or gravel is confirmed.
Deeper or rafts where fill, trees or drains control the design.
Main footing types
Strip or trench fill for most house extensions.
Pads and ground beams where you have steel frames or columns.
Raft slabs for soft or variable ground.
Piles only where soil conditions make normal footings uneconomic.
Pads and ground beams where you have steel frames or columns.
Raft slabs for soft or variable ground.
Piles only where soil conditions make normal footings uneconomic.
What Building Control check
Soil type and trial pit depth.
Tree influence zones on clay.
Distance to drains and sewers.
Existing footing depths at the house.
Ground bearing pressure and reinforcement details.
Clear notes on concrete grade and frost protection.
Tree influence zones on clay.
Distance to drains and sewers.
Existing footing depths at the house.
Ground bearing pressure and reinforcement details.
Clear notes on concrete grade and frost protection.
Information we need to design your foundations
Good information in makes the design smoother, reduces queries on site and helps Building Control sign off the foundations quickly.
Architect drawings in PDF or DWG
- Plans, elevations and sections showing the extension footprint, levels and roof form.
- We can work from architect PDFs or DWG exports if your designer prefers.
Site investigation or trial pits
- Soil type at depth, any fill and the level where firm ground is reached.
- Photos or notes of existing house foundations if they are exposed.
Trees, drains and services
- Location and size of nearby trees, even in neighbours’ gardens.
- Layout of private drains and any public sewers that might need build over consent.
Project address and timings
- Postcode, planning status and whether you already have Building Control appointed.
- When you hope to start digging so we can agree realistic design dates.
Foundations for new build houses
As well as extensions we design foundations for single new build houses, pairs of semis and small terraces on infill or garden plots.
Single new build homes
- Strip or trench fill foundations sized for soil bearing and settlement checks.
- Ground floor slabs, thickening at load bearing walls and steps at changes in level.
Semis, terraces and infill plots
- Shared wall foundations and local thickening at party walls or separating walls.
- Checks for adjoining buildings, retaining walls and boundary conditions.
Foundations and footings – frequently asked questions
Short answers to the questions we hear most often from UK homeowners planning an extension or new build.
What are the typical footings for an extension?
Most extensions use strip footings or trench fill footings depending on soil, depth, trees and drain locations. We confirm the right option when we review your drawings and site information.
How deep do footings need to be for a single storey extension?
On firm ground many rear extensions use depths around 1.0–1.4 m. On shrinkable clay with trees nearby the depth can increase to 1.5–2.5 m or more. We set the depth after reviewing the soil, trees, drains and any trial pit information.
Do you check drains and build over agreements?
Yes. We look at private drains and public sewers near the extension. If you need a build over agreement, we can design sleeves, lintels or diverted routes so the foundations and pipework work together.
Can you design our foundations without a site visit?
In many cases yes. With good drawings, trial pit photos and soil information we can design the footings remotely. If a visit is needed we will say so and include it in the quote.
Do you work with Building Control if they have questions?
Yes. Our fee includes reasonable follow up with your Building Control officer so any queries on footing depth, bearing pressure or reinforcement are answered quickly.
Do you also design foundations for new build houses?
Yes. We regularly design foundations and slabs for single new build homes, semis and infill projects. The process is the same – we review the drawings and soil, agree a fixed fee and deliver a Building Control ready pack.
Foundation design videos from our YouTube channel
For engineers and students who want to see the calculations behind the foundations, here are a few short tutorials.
Pad Foundation Design – Part 1
Simply Design Trench Fill Foundations
Eccentric Loading & Bearing Pressure
Homeowners who used us for extension and new build foundations
A few short comments from clients who asked us to design their footings, slabs and retaining elements.
Rear extension – Manchester
Trench fill footings on clay
“Clear notes for footing depth and concrete type. Our builder followed everything without issues and Building Control signed off the foundations first visit.”
Two storey extension – Birmingham
Footings near trees and drains
“They were very strong on tree influence and drain protection zones. We felt confident we had the right footing depth and did not dig any more than needed.”
New build house – Nottingham
Raft foundation on soft ground
“The raft solution was well explained and saved us very deep trenches. Building Control liked the detail and the contractor said the drawings were easy to follow.”
Request a structural engineer quote for your foundations
Use this form to outline your extension or new build foundations. Your enquiry goes straight to an engineer, not a call centre.
Tell us about your footings and foundations
Share a few details so we can review the drawings and confirm the right footing type, depth and a fixed fee for your foundation 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 extension or new build foundations forward?
If you are planning an extension or new build house and want clear structural calculations for footings, slabs and retaining walls, 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