Concrete sets in hours and stays set. On the day a slab is poured for a kitchen extension, six things stop being open questions and become the fixed conditions everything else works around.
The rest of the project stays adjustable. Worktops, cabinetry, tiling, paint and even glazing can be changed later at a cost that scales with the item rather than with the building.
What follows is what each of those six involves technically, and when it needs settling. Most are governed by a Building Regulations part, a water authority agreement or a structural calculation, which is why they belong in the design period rather than in a conversation on site.
One: The Drainage Layout
Drainage sits under the slab, so its geometry is decided before concrete arrives. More usefully, it is calculated rather than chosen.
The connection point to the existing drain has a fixed invert level. From there, Approved Document H sets the gradients a gravity drain has to run at, and those two constraints together determine where a sink can go.
For a 100mm foul drain serving a house, 1:40, meaning 25mm of fall per metre, is the commonly cited working standard. A shallower 1:80, or 12.5mm per metre, is acceptable where at least one WC discharges into the run, because the higher flow maintains the self cleansing velocity of roughly 0.75 metres per second. Sources differ on which of those is the minimum and which the preferred figure, but the practical point holds either way.
Too steep is also a problem. Beyond roughly 1:10, solids separate from the water and the drain strands them.
So the arithmetic runs one way. Fixed invert level, plus required fall per metre, gives you the maximum distance a new sink can sit from the connection. An island sink is the demanding case, because the run crosses the middle of the floor rather than following a wall, and every metre of that run consumes fall you may not have.
Several other Part H requirements land at the same moment:
- Surface water must be kept separate from foul drainage in new work
- Access points are needed at changes of direction, junctions and the head of long runs
- Cover depth is generally at least 450mm in gardens and 600mm under anything vehicles drive over
- A soakaway, where one is used, must sit at least 5 metres from any building and from the neighbouring boundary
None of those can be adjusted once the ground is closed and the slab is down.
Two: Whether There’s a Public Sewer Beneath You
This is a separate approval from planning, and the two are frequently confused.
A build over agreement from your water authority is normally required where work sits within 3 metres of a public sewer, or within 1 metre of a public lateral drain. The statutory basis sits in the Water Industry Act 1991, and sewers commonly run beneath back gardens in urban areas, which is exactly where a kitchen extension goes.
Two things make this trap common. Planning permission does not grant permission to build over a sewer. And the Transfer of Private Sewers Regulations 2011 moved most shared drains into public ownership, so a drain that looks private and sits entirely inside your boundary may well be a public sewer.
What the process involves:
- Thames Water quotes 21 days for its simplest class of application, with larger diameter sewers taking six to eight weeks alongside the build
- A CCTV survey is normally required before construction, and a second one afterwards on larger sewers
- Approval usually arrives with conditions rather than a refusal
- A reinforced concrete protection slab is a common condition, adding somewhere in the region of £1,000 to £3,000
- Foundations are typically restricted to strip or trench fill, with pad, raft and cantilever designs generally refused over a public sewer
There’s a long tail consequence to getting drainage signed off too. Unsigned drainage work surfaces during conveyancing, when a buyer’s solicitor asks for the completion certificate.
Three: The Finished Floor Level
A level threshold between kitchen and garden depends on a calculation made before groundworks.
The internal finished floor has to line up with external paving, with a drainage channel outside taking surface water away from the doors. To achieve that, slab level is set by working backwards from the door cill through every layer above it: insulation, screed, adhesive and the finish itself.
That means the floor finish has to be dimensionally fixed at slab stage. Substituting a finish 8mm thicker later produces a step, and the only remedy is lifting the floor.
The same arithmetic applies internally, where a new slab meets existing suspended timber floors in the original house. Those two levels rarely align without being made to.
Four: Foundation Type and Depth
Foundations are specified by a structural engineer from ground bearing pressure, with calculations going to Building Control under Part A. The specification responds to conditions rather than to preference.
| Site condition | Typical consequence |
| Stable ground, no trees nearby | Strip foundations |
| London clay, which shrinks and swells seasonally | Greater depth, more conservative design |
| Mature trees within a few metres | Deeper foundations or an engineered solution, with root protection zones affecting position as well as depth |
| Made ground or a sloping site | Trench fill, raft or piled foundations |
| Public sewer within 3 metres | Foundation type constrained by the water authority’s conditions |
Because cost varies so widely between those rows, a ground assessment before design carries more weight than most homeowners expect. It sets both the budget and what is physically achievable on the plot.
Five: The Width of the Structural Opening
Where the extension meets the existing house, a large opening is normally formed, and the beam spanning it carries the loads the wall previously took.
The engineer sizes that beam and specifies the section, expressed in a form such as 203 x 133 x 30 UB rather than as a generic steel. Bearing details and padstone sizes come with it, since the masonry at each end has to carry a concentrated load. Fire protection is usually required, with the specification following from the Part B assessment for the particular case.
Two consequences follow. Steel is fabricated to the drawing, with lead times commonly running two to five weeks from the point a fabricator receives certified calculations, so ordering late stops the site. And once the opening is formed and the beam is in, widening it means new calculations, new steel and new bearings.
Six: The Underfloor Heating Layout and Rooflight Positions
Two decisions that sit either side of the slab and share the same deadline.
Heating pipework or matting is buried in the screed. Zoning has to be decided in advance, because a zone boundary drawn in the wrong place cannot be moved, and the same applies to leaving areas unheated beneath fixed units.
The floor finish is part of that decision. Combined thermal resistance of finish and underlay should generally stay below 0.15 m²K/W, roughly 2.5 tog, or the system pushes heat into a covering that resists it. Surface temperature limits also differ by material, with engineered wood and vinyl typically capped lower than tiles or stone. A water system should pass a pressure test before being buried, and screed needs to reach a low enough moisture content before any finish is laid.
Rooflights sit within the roof structure, so their positions are settled when that structure is designed. Position interacts with joists and beams, and it also governs how far daylight reaches into the plan, since overhead glazing penetrates deeper than a vertical window of the same area. Roof form follows: a flat roof keeps sightlines from windows above less obstructed, a pitched roof buys ceiling height, and ceiling height is not revisitable.
Approved Document L also limits total glazed area to 25 percent of the extension floor area, plus the area of any existing openings built over, and rooflights count towards that alongside doors and windows.
How This Looks on a Real Project
Extension Architecture’s Coombe project in Kingston upon Thames included what the practice describes as a hidden commercial working kitchen, alongside a wellness suite, a 13 metre reflective entrance pool and a double height atrium, across a house taken from 200 to 600 square metres.
A commercial specification kitchen and a pool on the same site is a services problem before it is a design problem. Drainage capacity, falls, plant positions and duct routes all had to be resolved during design, because none of them could be adjusted once the structure was in.
The project also ran through a pre application consultation with the council before the full application was submitted and granted. That sequence is the point rather than the scale: the technical constraints were established first, and the ambition was fitted to them.
Working Backwards From the Pour
A ground assessment and a drainage investigation come first, because both can change the design rather than merely inform it. A build over application, where one is needed, goes in as early as possible given the timescales.
Structural design follows, covering foundations and the opening, which then allows steel to be ordered with enough lead time. Floor finish and threshold levels are settled alongside it, since slab level depends on them. Heating zones and rooflight positions complete the set.
None of these are difficult decisions individually. They become difficult when they are taken in the wrong order, or by whoever happens to be on site the week the digger arrives. A home extension architect sequences them so each is resolved by the time the next one depends on it.
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