Adding a single-storey kitchen extension, wrap-around living space, or two-storey addition is one of the best ways to expand living space and increase property value. However, every structural extension relies on a dependable foundation capable of transferring wall and roof loads safely into the ground without shifting or settling.
Choosing the correct concrete mix for your footings is not a matter of guesswork. Structural engineers, architects, and local building control officers work to strict British Standards (BS 8500 and BS EN 206) to ensure the mix matches ground conditions and structural design.
Understanding Domestic Footing Concrete Mixes
Concrete mixes for residential foundations are broadly categorised into “Designated Mixes” (such as GEN mixes) and “Standard/Designed Mixes” (such as C-classes):
1. GEN3 Concrete (C20 Equivalent)
GEN3 (General 3) is the most widely specified designated mix for unreinforced domestic foundations. It achieves a 28-day compressive strength of roughly 20 N/mm² (C20) and is formulated specifically for foundations where no steel mesh or rebar cages are present.
- Best for: Single-storey extension strip foundations, standard trench fill, mass concrete pads, and unreinforced garden boundary footings.
2. C25 / RC25 Concrete
C25 achieves a compressive strength of 25 N/mm² after 28 days. When steel rebar, starter cages, or reinforcing fabric are embedded in the trench, building codes require an “RC” (Reinforced Concrete) designation like RC25/30 to ensure the cement paste provides adequate cover and corrosion protection to the steel.
- Best for: Reinforced footings, two-storey extensions, stepped foundations on hillsides, or areas with variable ground bearing capacity.
Footing Options: Strip Foundations vs Trench Fill
How you pour your concrete depends on whether your project uses traditional strip footings or deep trench fill:
| Factor | Traditional Strip Footings | Trench Fill Foundations |
|---|---|---|
| Concrete Depth | 150mm – 300mm pad at the bottom | Filled up to ~150mm below finished surface |
| Masonry Required | Requires bricklayers to lay blocks deep inside trench | Minimal brickwork below ground level |
| Concrete Volume | Lower volume required | Higher volume required |
| Labour & Speed | Slow; high bricklaying labour costs | Very fast; trenches filled in a single morning |
Across suburban sites, trench fill has largely superseded traditional strip footings. Even though trench fill consumes more concrete, the massive savings in bricklaying hours, scaffolding, and sub-surface safety make it far more economical overall.
Ground Slabs for Extensions: Which Mix for the Sub-Floor?
Once your footings are poured and the below-ground blockwork reaches ground level, you will need concrete for your ground-bearing sub-floor slab:
- Mass Concrete Slabs: A standard C20 or GEN3 mix poured to 100mm over 1200-gauge polythene membrane, insulation boards, and compacted MOT Type 1 aggregate.
- Reinforced Slabs: If your structural engineer specifies steel reinforcing mesh (e.g., A142 or A193) to counteract ground movement, an RC25/30 or C25 mix is required.
Planning Volume and Delivery for Footings
Foundation trenches are notorious for absorbing more material than paper calculations predict. Trench bottoms are rarely laser-level, and trench walls often experience small collapses during excavation. Always calculate length × width × depth and add at least 10% to 15% extra to account for ground irregularities.
When arranging delivery, working with an on-demand supplier of concrete Stockport services prevents the stress of over- or under-ordering. Volumetric delivery ensures you batch the exact volume required directly into the footings, stopping the pour precisely when the concrete reaches your datum line.
Key Steps Before Placing Extension Concrete
- Schedule Building Control: Never pour concrete into footings until a Building Control officer has inspected the open trenches and signed off on ground stability.
- Protect Sub-Surface Pipes: If existing drainage or gas pipes cut across your footing trenches, fit compressible lagging or sleeve them with concrete bridging lintels to prevent structural cracking under settlement.
- Pump vs Chute Access: For rear extensions where the truck cannot back up to the trenches, arrange a ground line pump in advance to pipe the concrete directly into the footings within minutes.