An extension quote lives or dies on what's below ground and what's holding the roof up, both of which are invisible once the job is finished. If the specification doesn't name the foundation type, the drainage arrangement and the structural steel sizes, you are pricing blind.
This matters more on extensions than almost any other job type because ground conditions, existing drain runs and party wall boundaries vary house to house, and a generic quote can't have priced any of them properly.
Foundation type and ground conditions
The spec should state the intended foundation type — strip, trench-fill or piled — based on either a trial hole dug during survey or a stated assumption pending building control agreement. Extensions on clay soils near trees, or on sloping sites, often need deeper or piled foundations than a flat-rate quote assumes.
If the foundation depth is not yet confirmed, the quote should say so and explain how the price will be adjusted if building control requires greater depth once the trial hole is inspected — this is the single biggest source of extension cost overruns.
Drainage diversions
Most rear and side extensions build over or near an existing drain run. The spec should confirm whether a CCTV drain survey has been done, whether a diversion or a lintel-bridged section is planned, and whether Building Control or the water authority needs to approve the arrangement under the relevant regulations.
Diverting a shared or public sewer can require formal approval and should be flagged as a distinct line item with its own cost, not folded silently into groundworks.
Structural steel and openings
Any steel beam supporting the new opening or roof should be specified by size and grade from the structural engineer's calculations, not just called 'a steel beam'. The quote should state whether the engineer's fee, the calculations and building control's structural sign-off are included.
Padstones, bearing details and temporary propping during steel installation should also appear — these are frequently missing from cheaper quotes and add up quickly if priced as an afterthought.
Roof build-up and weathering
For a flat or low-pitch extension roof, the spec should name the covering system (single-ply, GRP, built-up felt), the insulation thickness and whether the roof is warm or cold constructed. For a pitched roof tying into the existing house, it should describe how the new and old roof coverings and flashings will match.
What cheap extension quotes usually miss
Party wall agreement costs, scaffolding for the full build duration rather than just the roof stage, and making good to the existing house where the new extension meets it are common omissions. So is the cost of temporary weatherproofing if the job spans winter months.
- Party Wall etc. Act notices and any required award
- CCTV drain survey and diversion costs
- Structural engineer's fee and calculations
- Scaffolding for the full build programme
- Making good the existing house at the new junction
Common questions
Why does an extension quote change after a trial hole is dug?+
Because foundation depth depends on ground conditions that can't be confirmed without physically digging and inspecting. If the ground is softer or the water table higher than assumed, building control may require deeper foundations, which changes the concrete and labour cost.
Do I need a structural engineer for a single-storey extension?+
In most cases yes, wherever a steel beam or new structural opening is involved. The engineer produces calculations building control will need to approve the design.
Is a drain diversion always needed for a rear extension?+
Not always — it depends on whether an existing drain run crosses the footprint. A CCTV survey before the quote is finalised tells you whether a diversion, bridging or no action is required.
Get an extension quote with foundations and drainage confirmed
We survey ground conditions and existing drain runs before quoting, so the foundation price is real, not assumed.