When you’re thinking about building an extension to your existing house, groundworks will be one of your first concerns. The simple answer is yes – proper groundworks are a legal requirement for any home extension, whether you’re planning a small side extension or a larger home improvement project.
At LGL Groundworks, we’ve been helping homeowners across Essex and Suffolk with their extension projects for over a decade. In this guide, we’ll explain everything you need to know about groundworks when building an extension and why they’re so important for your home improvement plans.
What Are Groundworks for Building Work?
Groundworks involve all the preparation work that happens before the actual building work begins on your extension project. This includes:
- Excavating and preparing foundations
- Installing drainage systems
- Setting up service connections (water, gas, electricity)
- Managing soil removal and site preparation
- Creating access routes for construction
Think of groundworks as the invisible foundation that your entire home extension builds upon. Without proper groundworks, even the most beautiful home improvement won’t stand the test of time.
Why Groundworks Are Essential When Building an Extension
It’s a Legal Requirement
UK building regulations make groundworks a legal requirement for all extension projects. The Planning Portal’s official foundations guide explains exactly what’s required. Your local authority must approve your groundworks to ensure your home extension is safe, stable, and won’t cause problems for your existing property. This applies whether you’re building under permitted development or have obtained full planning permission.
Special Considerations for Different Properties
If your existing house is a listed building or you’re working in conservation areas, your local planning authorities may have additional requirements for your groundworks. It’s always best to check with your local authority before starting any extension project.
Foundation Depth Requirements for Your Extension Project
The depth of your foundations when building an extension depends on several factors:
| Home Extension Type | Minimum Foundation Depth | Key Considerations |
|---|---|---|
| Single-storey side extension | 0.6m – 1.0m | Soil type, proximity to trees |
| Two-storey home extension | 1.0m+ | Higher load bearing requirements |
| Side return extension | 0.9m+ (typically clay soil) | Popular in urban areas |
| Extension in clay soil areas | 0.9m+ | Soil expansion and contraction |
| Building work near large trees | Up to 2.0m+ | Root systems and moisture changes |
For single-storey home extensions, the recommended foundation depth is generally 15cm deep on top of bricks or blocks, making the total depth around 1 metre. For two-storey building work, a concrete foundational depth of at least 200mm is required as it bears more load capacity than a single storey extension.
Soil Type Matters
Different soil types require different approaches:
Clay Soil: Clay soil expands and contracts seasonally according to change in weather. Low plastic clays usually expand up to 700mm whereas clay soil with medium plasticity expands up to 900mm. This means foundations in clay areas often need to be deeper than expected.
Sandy/Gravel Soil: These soils are more stable and typically allow for slightly shallower foundations due to better drainage and less movement.
Rock-based Ground: Offers excellent load-bearing capacity and may allow for shallower foundations in some cases.
Key Factors That Affect Your Extension Project Groundworks
1. Your Existing House Foundations
When building an extension, you must make sure the excavation for the new foundation doesn’t undermine your existing property. It’s good practice to dig at least to the same depth as the bottom of your existing house’s foundation.
Your home extension’s foundations must work well with your existing house’s foundations. This often means matching or going deeper than your current foundations.
2. Drainage and Services for Your Building Work
If a drain or sewer is within the area covered by that 45 degrees area there is a risk that it could be affected by the load from the foundation and possibly crack. Therefore, the foundation excavation should normally be at least to the same depth as the bottom (invert) of the deepest part of the drain, sewer or its trench.
Existing drains, sewers, and service lines must be considered during planning your extension project. Sometimes these need to be diverted or protected during your building work.
3. Trees and Landscaping Around Your Extension Project
The presence of trees in clay soil areas can mean foundations need to be significantly deeper than might be first expected. Large trees near your home extension can affect foundation depth requirements significantly. This is especially important for side extensions in urban areas.
4. Site Conditions for Your Home Improvement
Factors like:
- Sloping ground (may require stepped foundations)
- Previous building work or filled ground
- Local water table levels
- Proximity to neighbouring properties
The Groundworks Process for Building an Extension
1. Site Survey and Planning for Your Extension Project
Before any digging begins, a thorough site survey identifies potential challenges and determines the best approach for your specific home improvement project.
2. Excavation Work
Generally the topsoil is taken away and good undisturbed ground is found i.e. ground that has not been built on. The foundation cannot be poured until undisturbed ground has been found.
3. Foundation Installation for Your Building Work
Depending on your site conditions, this might involve:
- Strip foundations (most common for home extensions)
- Trench fill foundations
- Pad foundations
- In challenging conditions: piled or raft foundations
4. Drainage and Services for Your Home Extension
Installation of new drainage connections and service routes to support your extension project.
5. Building Control Approval – A Legal Requirement
Local building inspectors play a crucial role by visiting the site and signing off on the trenches before the next phase of your building work. This approval is a legal requirement for all extension projects.
Cost Considerations for Your Home Extension
Foundations for a single storey extension project typically cost between £110 and £165 per m2, depending on the type. However, costs can vary based on:
- Soil conditions and required depth
- Access to the site
- Need for drainage diversions
- Local ground conditions
- Size and complexity of your home improvement
When Professional Help Is Essential for Building an Extension
While it might be tempting to cut corners, proper groundworks for any extension project require professional expertise. This is especially true if you’re working on a listed building or in conservation areas where your local planning authorities have strict requirements.
At LGL Groundworks, our experienced team understands the complexities of extension project groundworks and can help ensure your home improvement starts on the right foundation. We work with homeowners, local authorities, and local planning authorities to ensure your building work meets all requirements.
Whether you’re planning a simple side extension or a complex side return extension, we understand that every extension project must comply with building regulations.
We provide comprehensive groundworks services that cover everything from initial site assessment through to final building control approval for your extension project.
Conclusion
Groundworks aren’t just recommended for home extensions – they’re a legal requirement for all building work. Proper groundworks ensure your home extension is safe, stable, and complies with all building regulations. While the work happens largely underground and out of sight, it’s the foundation upon which your entire extension project’s success depends.
Whether you’re building under permitted development or have planning permission from your local authority, proper groundworks are essential. This applies to all types of home improvements, from simple side extensions to complex side return extensions.
With over 10 years of experience in Essex and Suffolk, LGL Groundworks has the expertise to handle your extension project groundworks professionally and efficiently. We understand local soil conditions, building regulations, and the importance of getting the foundations right the first time for any home improvement.
For more information about our comprehensive groundworks services for your extension project, visit our groundworks page or contact us today for a consultation about your home extension plans.
Frequently Asked Questions
How deep do foundations need to be when building an extension?
For most single-storey home extensions, foundations need to be at least 1 metre deep, though this can vary based on soil type and local conditions. Clay soils typically require deeper foundations (0.9m+), while the presence of large trees nearby may require foundations up to 2 metres deep or more. If you’re planning a side extension or side return extension, your local authority will specify the exact requirements for your extension project.
Do I need building control approval for extension project groundworks?
Yes, building control approval is a legal requirement for all extension project groundworks. Building control officers must inspect and approve the excavated trenches before concrete can be poured. This ensures your home extension meets building regulations and is safe for building work. This applies whether you’re building under permitted development or have full planning permission.
How long do groundworks take for a typical home extension?
The duration depends on the size and complexity of your extension project, but most residential home improvement groundworks take between 1-3 weeks to complete. Factors like soil conditions, drainage requirements, and weather can affect timing. Your groundworks contractor will provide a more specific timeline based on your building work requirements and whether you need to comply with building regulations for listed buildings or work in conservation areas.
What happens if we discover poor soil conditions during our extension project?
If poor soil conditions are discovered during your building work, such as soft or unstable ground, alternative foundation solutions may be needed. This could include deeper excavations to reach stable ground, piled foundations, or raft foundations. A structural engineer may need to be consulted to determine the best solution for your specific home extension. Your local authority will need to approve any changes to ensure your extension project still meets all building regulations.
