Unlike a simple garden fence or a raised planter, a retaining wall must battle the immense forces of gravity and hydrostatic pressure every single day of its life.
Rushing into construction without adequate preparation is a recipe for disaster—leaning walls, cracking mortar, or even total collapse. To ensure your project stands the test of time, you need a solid plan before you lift a single shovel.
That’s why the team here at PPC are sharing the things you must do before building a retaining wall within this blog.
Foundation first: check the soil
Before you design anything, you must know what lies beneath your potential retaining wall. The type of soil on your property dictates the design of the wall, the depth of the foundation, and the type of reinforcement required.
Sandy soil generally drains well but has low cohesion, meaning it can crumble easily during excavation. However, clay soil is the most problematic for retaining walls. Clay expands when wet and shrinks when dry, creating significant movement and pressure against the back of the wall.
If you are dealing with heavy clay, the risk of structural failure increases significantly if drainage is not prioritised. The heavy, water-logged soil pushes against the wall with tremendous force.
Before proceeding, it is vital to understand how clay soil affects retaining wall stability. If you are unsure of your soil type, a simple DIY soil test kit or a consultation with a geotechnical engineer is a wise investment.
Understand the legal requirements for retaining walls
While many retaining walls can fall under ‘Permitted Development’, there are strict limitations.
Generally, you will need planning permission if:
- The wall is over 1 metre high and next to a highway (this includes footpaths and roads).
- The wall is over 2 metres high elsewhere on your property.
- Your home is a Listed Building or located in a Conservation Area (rules here are much stricter).
Even if your wall does not require planning permission, it must still be structurally safe. Retaining walls are dangerous if they collapse. For higher walls, Building Regulations approval may be required to ensure the structure is engineered correctly. Ignoring these rules can lead to enforcement notices requiring you to tear the wall down.
Is there a “surcharge” load?
A common mistake is assuming the wall only needs to hold back the soil immediately behind it. You must consider what is happening above and behind the wall. This is known as the “surcharge load.”
Ask yourself:
- Will there be a driveway or parking space above the wall?
- Is there a heavy structure, like a summer house or hot tub, nearby?
- Does the slope continue upwards steeply above the wall?
If you are building near farm tracks or areas with heavy machinery, the requirements change drastically. You can read more about what types of retaining walls work best on farms to understand how heavy loads influence material choice. Failing to account for these extra weights is one of the primary reasons walls bulge and fail.
Drainage is a non-negotiable
If there is one rule to memorise, it is this: Water is the enemy of the retaining wall.
When water accumulates behind a wall, it creates hydrostatic pressure. This pressure is often far greater than the weight of the soil itself. If the water has nowhere to go, it will eventually force its way through the path of least resistance—usually by pushing your wall over.
Your design must include:
- Backfill: A zone of clear, crushed stone (gravel) directly behind the wall to allow water to drain downwards.
- Drain pipe: A perforated agricultural drain pipe at the base of the wall to carry water away to a suitable outlet.
- Weep holes: Small gaps in the wall face to allow trapped water to escape.
For a deep dive into the mechanics of this, read why drainage matters for retaining walls and how to get it right. Additionally, if you are building a concrete block wall, you need to consider how to protect the structure itself from moisture ingress. Check out things to know when waterproofing a retaining wall to prevent damp issues later on.
What material should you use?
The material you choose affects not just the look of the wall, but its longevity and cost.
Timber sleepers
Timber is popular for a rustic, natural look. It is generally easier for DIY’ers to handle. However, wood eventually rots.
- Learn more: How to make a sleeper retaining wall.
Concrete blocks
Hollow or solid concrete blocks are incredibly strong and durable. They are ideal for walls that need to last for decades. They can be left raw for an industrial look or faced with stone/render.
- Learn more: Concrete blocks: An eco-friendly building material.
Gabions
Wire cages filled with stone are excellent for drainage and are increasingly popular in modern garden designs.
If budget is your primary concern, you need to weigh up the cost of materials versus the lifespan of the wall. We have broken this down in our guide on what is the cheapest way to build a retaining wall in the UK.
Conduct a realistic DIY assessment
We all love a “Grand Designs” moment, but a retaining wall is a structural entity, not just a decorative feature. So when should you consider calling a professional?
You can probably DIY if:
- The wall is under 1 metre high.
- The ground is relatively flat and stable.
- You are physically fit (it involves moving tonnes of soil and stone).
- You are using user-friendly materials like sleepers or small blocks.
You should hire a pro if:
- The wall is over 1 metre.
- The slope is steep or the soil is difficult (clay/sand).
- There are surcharge loads (driveways/buildings).
- You lack excavation machinery.
Before you pick up a sledgehammer, read our honest assessment: Can I build a retaining wall by myself?. Remember, a failed wall is twice as expensive to fix as a wall built correctly the first time. In fact, experts warn this one retaining wall error will cost you thousands, and it is usually related to cutting corners on foundations or drainage.
Plan for access and waste
Logistics can kill a project schedule. Building a wall involves two major movements of material:
- Excavation: You will be digging out tonnes of earth for the footing and the cut of the bank. Where will this soil go? Do you need a skip? Can a grab lorry access your property?
- Materials In: Blocks, gravel, sand, and cement are heavy. If you have a terraced house with no side access, how will you get materials to the garden?
Finalise the design
Once you have assessed the soil, the law, the drainage, and your budget, sketch it out.
- Mark the line of the wall with spray paint or string.
- Check the levels.
- visualise the finished height.
If you are looking for inspiration on how to finish the wall to make it a feature rather than just a barrier, explore our retaining wall ideas that completely transform your garden.
Summary – Checklist
Action Item | Why it matters |
Test Soil | Determines foundation depth and drainage needs. |
Check Planning | Avoids legal trouble and enforcement notices. |
Calculate Loads | Ensures the wall won’t collapse under pressure. |
Plan Drainage | Prevents water build-up (the #1 cause of failure). |
Choose Material | Balances aesthetics, budget, and longevity. |
Assess Access | Ensures you can get materials in and waste out. |
Building a retaining wall is a serious undertaking, but with the right preparation, it adds immense value and usability to your property. If you take the time to tick these boxes before you start, you will build a structure that stands firm for generations.
Ready to get started? If you have your plan in place and are ready to choose your materials, browse our range of high-quality concrete blocks and lintels. Or contact our team at PPC Concrete Products for advice on the best products for your specific load-bearing needs.
