Construction, Extension
How a Rear or Side Extension Is Built — Full Construction Sequence Explained (AGC System)
Dec
Everything below is taken from our AGC construction system and supported by the Sequence of Extension Work document you provided.
1. Site Preparation & Removal Works
The first step is preparing the rear garden or side space by removing existing slabs, tiles, or old surfaces.
Your PDF clearly shows this stage on page 2: removal of slabs and clearing the working area.
This step ensures clean, safe access for excavation and structural formation.
2. Excavation for Foundations
Shown on pages 3 and 4, deep trenches are excavated to the required depth—typically 1m to 1.5m depending on soil conditions and nearby structures.
Correct excavation is critical because it determines the stability of the entire extension.
3. Leveling & Compaction
Once the trenches are dug, the ground is levelled (page 5) and compacted (page 6).
This avoids uneven settlement and ensures the foundation concrete performs as designed.
4. Pouring the Foundation Concrete
Your document shows concrete placement on page 7.
This is usually a strip foundation, sized by a structural engineer to support the load of the new walls, roof, and openings.
The AGC system uses a standardised reinforced strip foundation for most extensions.
5. Base Blocks & Wall Construction
Pages 8 and 9 cover the construction of the foundation block courses and the cavity wall system.
At this stage:
- Brick outer leaf
- Insulated cavity
- Dense block inner leaf (usually 7.3N or 10.4N blocks)
This structure forms the backbone of thermal performance and structural stability.
6. Water, Plumbing & Electrical Provisions
Pages 10 and 11 show how first-fix plumbing and electrical systems are embedded.
Extensions always require:
- Hot and cold water feeds
- Drainage links
- New electrical circuits
- Lighting and socket positions
These must be coordinated before the floor structure is closed.
7. DPM, Insulation & Concrete Slab
Pages 12–14 display the full build-up:
- DPM (damp-proof membrane)
- Floor insulation boards
- Concrete slab screeded to level
This creates a warm, dry, structurally sound floor ready for finishes.
8. Structural Steel & Openings
Page 16 illustrates the most important part of any modern extension: structural steel installation.
This includes:
- Removal of load-bearing rear walls
- Installation of RSJs or box-section steels
- Temporary propping
- Connection of new steel to existing structure
This is what creates the large open-plan kitchen/dining space that increases property value.
9. Roof Frame & Warm Roof Construction
Page 30 demonstrates roof installation.
AGC typically uses:
- Timber joists (50×150 to 50×225 depending on span)
- Structural firrings for pitch
- Warm roof insulation above joists
- GRP or felt waterproofing
Your skylight installation is also shown on page 31.
10. Internal Walls, Drylining & Plastering
Pages 15, 17, 18, 19 show partitions, plasterboard installation, skimming, and sanding.
This is where the extension starts looking like a real room.
11. Windows, Bifold Doors & Glazing
Pages 22 and 23 show the installation of new windows and large bifold doors.
These are responsible for:
- Bringing natural light
- Creating indoor–outdoor flow
- Increasing thermal performance
- Driving the resale value
This is one of the biggest contributors to the “wow factor”.
12. Floor Heating, Screed & Final Flooring
Pages 24–29 show underfloor heating, self-levelling screed, and final floor installation.
This stage completes the internal comfort experience.
13. Decorating, Fixtures & Final Finish
Pages 21, 27, 28, 34, and the final page illustrate decorating, lighting installation, and finishing touches.
This is the final polish before handing over a finished space ready for use.
14. The Finished Product
Page 35 in your document shows the final outcome: bright, spacious, high-performance living.
This is exactly the type of finished quality that helped your client sell their property for £1.25 million, creating £600,000 in uplift.
Why This Sequence Matters
Most homeowners only see the final extension, but value is created in:
- The engineering behind the foundations
- The structural steel calculations
- The correct cavity wall build-up
- The warm roof design
- The sequencing that avoids defects
This is why AGC extensions consistently outperform the market — the system is predictable, engineered, and optimised.
Thinking About Extending Your Home?
If you have a rear extension, side extension, or full house redesign in mind, understanding the correct sequence of work is the first step to building value — not problems.
Our AGC System handles:
✔ Structural design
✔ Foundations
✔ Block & brick walls
✔ Roofing & skylights
✔ Glazing
✔ Full build & project sequencing
You want an extension that adds value?
Contact us now.