Who this guide is for
- Owners and self-builders issued a structural package they have not seen before
- Anyone comparing what was drawn with what is being built
- Buyers reviewing the documentation for a recently built or altered house
- Owners preparing questions for a structural drawing review meeting
- Anyone planning an alteration who has been given the original foundation drawings
The plan is a setting-out document first
Before it describes any element, the foundation plan says where things go. Positions, dimensions and reference lines are what the rest of the drawing hangs from, and the library is repeatedly blunt that setting out and sizing are a single decision rather than two.
That is why a wall moved during design development is a structural change rather than a tolerance to absorb: on a strip it can push the load off centre, and on a raft it can leave a downstand carrying nothing and a thin area carrying a wall.
Levels are the other half of the plan
Foundation drawings carry levels because a foundation is a three-dimensional arrangement described on a flat sheet. Founding levels, changes of level and the positions of steps are all expressed as annotations rather than as shapes, and a plan with steps on it is describing a load path that changes as it goes.
External and internal finished levels appear for the same reason. The relationship between external ground, the damp-proof course and the internal floor is what decides what the barrier at the wall base is being asked to do, and that relationship is fixed on the drawings rather than on site.
The section is where the layers appear
A plan cannot show order, and order is most of what a substructure is. A section through the foundation and the ground floor shows what sits on what: the formation, the make-up, where the barrier runs, where insulation sits relative to the slab, and how the wall meets the floor at the perimeter.
That corner is worth finding on every set. The ground-bearing slab entry describes the perimeter as the place where structure, damp control and thermal continuity are resolved at once, and where drawings from different disciplines have to agree.
- Which layers sit above and below the structural element
- Where the floor barrier is turned up and what it laps into
- Where insulation stops, and what carries it past the junction
- Whether the slab bears on the foundation or is kept clear of it
Reinforcement notes describe an intention, not a picture
Steel is generally described rather than drawn to scale, and the notes say what the arrangement has to achieve rather than showing every bar. Where the steel is lapped, stopped or interrupted determines where an element can be treated as continuous, which is why those positions are annotated so carefully.
Two positions carry more weight than their size suggests. Starter bars are reinforcement deliberately left projecting so the next pour can be continued from them, and they cannot be threaded into hardened concrete afterwards. Kickers are short concrete upstands that fix the line of the wall above and are not the structural connection, which the steel is.
Hold-down and cast-in items are the least forgiving thing on the sheet
Where a frame lands on a foundation, the drawing carries the positions of cast-in bolts, anchors or sockets. These are the only parts of a foundation that have to line up with something fabricated in a workshop, which makes their position the least forgiving setting-out in the assembly.
The drawing may also state the base condition the frame design assumed. Whether a column base is treated as free to rotate or held rigidly changes what the foundation has to resist, so that note is describing a structural assumption rather than a detail.
Penetrations belong on the drawing, not on site
Every drain and incoming service crosses the foundation somewhere. Where an element spans, an opening is a hole through a beam rather than through a lump of concrete, so its size and position belong to the beam design. Where a plate is cast, a hole cut afterwards damages the structure and the barrier together.
A foundation drawing with no service positions on it is a drawing that has not yet had that conversation, and that conversation is easier before the reinforcement is set out.
What the drawing deliberately leaves open
Some things are not on the sheet because they cannot be. The formation is not known until it is exposed, and what is found there can change the level. Pile positions are recorded after installation because deviation is absorbed in the cap, not in the ground.
Notes referring decisions to site inspection are therefore not gaps. They are the drawing saying that a judgement belongs to the person standing in the excavation, and they usually name who that is.
Revisions matter more here than elsewhere
Because the substructure is built early and buried quickly, a superseded foundation drawing can be the one somebody worked from. Checking that the sheet in your hand is the sheet that was built from is part of reading it, and the revision record is what makes a later alteration assessable.
Reading a foundation drawing
- 1Confirm which revision was actually built from, and keep the superseded sheets separately.
- 2Find the setting-out reference lines and check the building is dimensioned from them.
- 3Identify every founding level and every step shown on the plan.
- 4Find the stated external ground level and internal floor level.
- 5Find the section through the perimeter and follow the layers from formation upward.
- 6Identify where the floor barrier turns up and what it laps into.
- 7Identify where insulation sits relative to the structural element.
- 8Read the reinforcement notes for where steel is lapped, stopped or projecting.
- 9Locate any cast-in items and note who reconciles them with fabricated components.
- 10Locate every drainage and service penetration shown, and query any that are missing.
- 11Note every instruction referring a decision to site inspection, and who makes it.
- 12Take the sheet to the drawing review meeting rather than a summary of it.
Common mistakes to avoid
- Reading a foundation drawing as though it were an architectural plan, when it is answering different questions.
- Treating a moved wall as a minor change, when setting out and sizing are a single decision.
- Assuming reinforcement drawn schematically is drawn to scale.
- Confusing a kicker with the starter bars beside it, when one positions the formwork and the other carries force.
- Working from a superseded revision because the substructure was built early.
- Accepting a drawing with no service penetrations on it and resolving them on site.
- Reading a note referring a decision to site inspection as an omission rather than as a deliberate handover.
- Filing the structural drawings apart from the ground investigation they were designed against.
When to involve a professional
- Whether a foundation design suits a particular building and its ground is a determination for the qualified structural engineer responsible for it.
- Reading a drawing does not qualify anyone to alter what it describes; changes to a substructure are design decisions.
- Cast-in item positions and their reconciliation with fabricated components are coordinated between the frame designer and the foundation designer.
- This page states no dimension, level, bar size, lap, projection or tolerance of any kind, and none can be inferred from it.
Frequently asked questions
Questions readers ask about this topic
Why does the foundation drawing look so sparse?
Because much of what it describes is annotation rather than shape: levels, steps, reinforcement intent, penetration positions and instructions referring judgements to the site. A section through the perimeter usually carries more information than the plan it accompanies.
What is the difference between a kicker and a starter bar?
A kicker is a short concrete upstand cast with the pour beneath that locates the formwork for the wall above. A starter bar is reinforcement left projecting so the next pour can be reinforced continuously. One positions the mould; the other carries force.
Why are service positions on a structural drawing?
Because every drain and service crosses the foundation. Where an element spans, an opening is a hole through a beam and belongs to the beam design; where a plate is cast, a hole cut afterwards damages the structure and the barrier beneath together.
Does the drawing tell me how deep the foundation will be?
It will state an intended founding level, but the formation is not known until excavation exposes it, and what is found there can change that level. Notes referring the decision to site inspection are describing a judgement that belongs in the excavation.
Should I keep the drawings after the build?
They are what any later alteration is assessed against, alongside the ground investigation and the record of what was found at formation. A substructure cannot be re-examined, so the drawings and the revision record are most of what a future owner has.
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