House Extension: Demolition, Groundwork, and Digging New Foundations

Building an extension yourself is one of the most demanding home improvement projects you can take on. Before any walls go up or new rooms take shape, the first stage involves demolition, clearing old structures, tackling unexpected obstacles below ground, and setting out new foundations accurately. This article walks through a full first-phase process from removing an outdated conservatory to excavating new footing trenches ready for concrete.

Removing the Old Structure

The existing rear conservatory must be dismantled before any new construction can begin. The safest and easiest way to do this is to strip out all glazing first. With the glass removed, the framework becomes lighter and far easier to handle. Old aluminium or PVC frames often degrade over time, so most components break down with simple hand tools or can be cut free if fasteners have rusted.

Once the doors, roof sections, and wall framing are removed, the original structure comes apart quickly. Even a medium-sized conservatory can be fully dismantled in a few hours with organised work. This clears the footprint for the new extension and exposes the slab and footings underneath.

Breaking Out the Old Walls and Slab

With the conservatory dismantled, the next step is to remove the dwarf walls and the concrete pad that supported the structure.

Removing the old walling

Using basic demolition tools such as an SDS drill, a mallet, and chisels, the inner skin of blockwork can be collapsed inward. The outer skin can then be tipped over and removed. Any clean, matching bricks worth reusing are set aside. Salvaged bricks can be invaluable later when adjusting openings or matched work is required.

Tackling the slab and footings

Removing the concrete pad exposes deeper concrete below it. Many conservatories sit on shallow foundations, but older or upgraded ones may have substantial footings. In this case, a thick section of concrete sits below the slab and proves resistant to handheld breakers.

Where slab thickness is manageable, electric breakers can reduce it to chunks. For deeper reinforced footings, a mechanical solution is usually needed. Small excavators with hydraulic breakers or standalone hydraulic power-pack breakers make this work safer, faster, and more controlled. By breaking sections into smaller pieces and levering them up, the area can be cleared efficiently.

Dealing With a Hidden Drain Cover

Demolition often uncovers surprises, and one of the most significant is an inspection chamber or manhole cover buried beneath floor coverings. Whether this becomes a major problem depends on whether the chamber serves only the property or is part of a shared or public drainage system.

Private chamber

A private run means the chamber can be moved or redesigned as part of the project. It still adds work, but it does not delay the entire build.

Shared or public chamber

A shared system normally requires approval from the water authority, inspections, and potentially an approved contractor. This can add weeks or months to a schedule.

Once the cover is carefully lifted and the run is confirmed as private, the extension can continue without a pause. The chamber will still need redesigning later when new drainage is installed, but it no longer threatens the project’s progress.

Preparing the Ground for New Foundations

Before digging new trenches, the ground must be cleared, levelled, and compacted. Excavators can be used to remove loose soil or backfill voids left by old footings. A small compactor plate ensures the surface remains stable while trenches are laid out.

At this stage, it also becomes clear whether any existing footings can remain in place. If a former footing lies outside the new foundation line, it may be left undisturbed. If it sits directly beneath a future wall, it must be removed, regardless of depth or condition.

Setting Out Reference Levels

Accurate levels are essential for the remainder of the project. Two heights are particularly important:

• Damp-proof course (DPC) height

• Finished floor level of the existing house

A rotary or cross-line laser with a staff and receiver allows these levels to be transferred accurately. The DPC is located on the existing house wall, and that height is transferred to a stake driven into the ground. A screw is set at that point to create a fixed reference.

The finished floor level is then measured and marked on the same stake. The difference between floor level and DPC allows the builder to calculate every future height: foundation concrete, oversite build-up, insulation, screed, and the final internal finish.

Once these two points are established, the stake becomes the project’s permanent datum. Any level anywhere on site can be checked or recreated by simply placing the staff on the reference screw and adjusting accordingly.

Ensuring the Existing Structure Is Aligned

The new extension must tie neatly into the existing house and any existing extensions. If the structures are not parallel, future roof timbers or wall runs may end up tapering or misaligned.

Measurements are taken from the back wall of the house to the existing rear extension at two separate points. Matching measurements indicate a parallel structure. To check squareness, the 3-4-5 rule can be used: three metres along one wall, four metres along the other, with a five-metre diagonal confirming a right angle.

These checks ensure the new extension follows the correct geometry and aligns properly with the house.

Installing Profiles and Setting Out Trench Lines

Temporary timber profiles allow precise and repeatable layout of the new foundation trenches. These are placed beyond the area to be excavated and fitted with vertical boards to hold string lines.

Determining wall thickness and centre line

A typical cavity wall is 300 mm thick:

• 100 mm outer leaf

• 100 mm cavity

• 100 mm inner leaf

By measuring 150 mm from a known point, the centre of the wall can be marked on the profile boards. String lines stretched between the profiles create a clear guide that represents the wall’s centre line on the ground.

A trench width of around 600 mm ensures there is enough concrete either side of the 300 mm-wide blocks that will form the first course. Once the centre line is known, it becomes easy to mark where the trench edges should be.

Spray paint is used beneath the line on the ground to give the excavator operator a visible guide when digging.

Excavating the Footings

With lines marked and levels set, the excavation begins.

Depth requirements

Drawings typically specify the required depth for foundations. In many domestic builds, footings need to be around one metre deep, but this varies depending on soil conditions, local regulations, and structural engineering requirements. Using the laser and the datum stake, the required depth is measured precisely. As the digger removes soil, the staff and receiver confirm the exact depth at each section.

Oversite excavation

At the same time, the area inside the footprint is taken down to its required depth for the floor build-up. In this case, the oversite level is about 400 mm below the finished floor height, allowing room for hardcore, concrete, insulation, and screed.Inspections and Approval

Once the trenches are dug, building control must inspect them before any concrete is poured. All visits for the project are typically covered by a single fee. During the inspection, the officer checks:

• Correct trench depth

• Adequate width

• Soil conditions

• Proper clearance around drainage sections where needed

If future drainage will be completely replaced, some temporary issues in the ground are not a concern. Once approved, the project is cleared to proceed to the next stage: pouring concrete.

What Comes Next

With demolition complete, obstacles removed, levels set, and trenches dug, the next phase can begin. Concrete can be poured, the walls can be built up to the damp-proof course, and the new drainage layout can be installed. These steps form the foundation—literally and figuratively—for everything that comes after.

The first phase may not involve visible progress above ground, but it is the most important stage for long-term stability. A well-prepared foundation ensures the extension performs properly and aligns perfectly with the existing structure.