Underground car park BOQ: the chapters that do not exist above ground, and the traps found only below
An underground car park BOQ (bill of quantities) is a separate bill, even when the car park sits under an ordinary residential building. A basement has systems that do not exist above ground, whole chapters that start and end below ground level, and areas that behave differently from a typical floor. On most projects it is also the place where a single error in a formula costs the most money.
Why an underground car park BOQ differs from a bill for the floors above
The first reason is the denominator. The basement area is not the area of the typical floor: the basement extends beyond the building line, or is smaller than it, and it has its own levels. Any ratio calculated on the wrong area, for example concrete volume per m² or rebar weight per m³, is wrong from the start. In my work the basement gets its own sheet, with its own area, and a check that no above-ground item has leaked into it.
The second reason is the chapters. A car park has shoring, deep excavation, waterproofing in series of details, smoke extraction, a sump pit, a water reservoir and firefighting supply, parking bay markings and barriers. On the upper floors none of these exist. The page construction BOQ describes the chapter map of an ordinary building; this page describes what is added below.
Shoring and excavation: the chapter most prone to errors
- Shoring piles. Measured in linear metres as the number of piles times their length, and separately the concrete in them in m³, by the cross-section area. The cross-section of a pile half a metre in diameter is about a fifth of a m². A result double that is a sure sign of a bracket error in the formula, and it has actually happened to me.
- Ground anchors. A count: the perimeter length divided by the spacing. Multiply instead of dividing and you get m² instead of units, and the size of the error depends on the spacing. A count plus m² is not a number.
- Capping beam. Two alternatives that must not be added together: the volume of a single pile cap times the number of piles, or a continuous beam along the perimeter. The gap between them can reach tens of per cent. Decide on one and record the decision.
- Pile versus shoring wall. The same physical line cannot be measured both as a row of piles and as a continuous wall. A warning sign: the two volumes come out in the same order of magnitude.
- Excavation. The excavation depth is the natural ground level minus the bottom level. Without a natural ground level this is missing data, not an estimate. Under the General Specification for building works (Israel's inter-ministerial specification), the excavation volume at a shoring face is measured from the line tangent to the face of the piles, and working space is not measured. More on the items on the page excavation and shoring BOQ.
Three questions are worth asking of the soil report and the hydrological opinion before writing anything: the quality of the groundwater, because brackish water can change the concrete specification of the whole basement; the distance in metres between the groundwater and the bottom of the excavation, because a small distance usually triggers a recommendation for a piezometer and dewatering; and whether there is a separate soil report, because it is the source of the soil profile. All of this has to be checked against the project's soil report and with the geotechnical consultant and the structural engineer.
Structure, waterproofing and finishes in the basement
In the basement structure, the bottom concrete slab is measured by its area times its thickness, and that area is not the total built area. The basement walls are measured as length times thickness times net height, and the top slab of the basement is measured separately when its thickness differs, because it carries the external works. The rebar ratio in the basement is usually higher than the average for the building, so a single ratio for the whole building skews both parts in opposite directions.
Waterproofing comes in series of details: for the basement slab, for the walls in two height stages, for the protection of the waterproofing and the clean backfill, for the basement roof slab under the external works and for the lift pit, plus gas seals and pipe penetrations that are counted from the model. Two questions from the waterproofing specification booklet change the price and the measurement boundary: who does the preparation, the frame contractor or the waterproofing contractor, and whether the measurement is by developed area or by plan projection. In the finishes, power-floated concrete is measured in m² and the slab under it in m³, and they must not be added together. Painting walls "on both sides" needs a multiplication by two in the formula, and the number of parking bays to mark comes from the building permit, not from dividing the car park area by a round number.
| What | Where it gets counted twice |
|---|---|
| Pile versus shoring wall | The same physical line in two items |
| Backfill | In earthworks and again as waterproofing protection |
| Concrete slab versus power-floated finish | m³ and m², two items that must not be added together |
| Roof slab waterproofing versus external paving | In the basement bill and in the external works bill |
| Damper versus smoke extraction fan | Two components entered as one |
| Fire door | In the metalwork chapter and again in the fire safety chapter |
| Water reservoir waterproofing | Against the basement slab waterproofing |
The rule. For each trap, decide who measures it, and record the decision in the line itself. A bill with the decision written into it saves the argument when the payment account is measured. On the difference between the quantities, see the page physical quantity, BOQ quantity and order quantity.
Systems that exist only below ground
These include mechanical smoke extraction with dampers and fans, where the acoustic silencer is a separate item; fire detection and carbon monoxide detection; sprinklers by area divided by the coverage area per head, where the coverage depends on the hazard classification; a fire pump room with an acoustic door; a sump pit and pumps; a water reservoir in m³, whose waterproofing is a separate item; railings, column guards and wheel stops in linear metres; and a car park gate and barrier. All of these come from the consultants' bills, and in the contractor's bill they are collected as they are, not reinvented.
What a professional quantity surveyor adds here
Whoever measures the basement by the drawing alone gets piles, walls and a slab. A quantity surveyor who comes from the field, an engineer or a registered building technician who worked on site, like Yaakov Avital, reads the shoring plan together with the soil report and the hydrological opinion, and asks what is missing between them. For example: the shoring plan shows a row of anchors at a certain spacing, while the soil report recommends anchors on only some of the faces and struts on the rest. Measuring by the drawing will put anchors along the whole perimeter. A professional check flags a question to the structural engineer and the geotechnical consultant, and they decide. The result is a flag in the bill, not a correction by the quantity surveyor.
In a car park and basement, the quantity takeoff includes a lot of work that is not visible at the end: shoring, waterproofing, drainage and temporary supports. When the quantity takeoff leaves them out, they come back as variations.
Frequently asked questions
Why prepare an underground car park BOQ separately from the building?
Because the car park has its own area, its own chapters and its own systems, and because the shoring, waterproofing and car park systems contractors are usually separate subcontractors. A separate bill makes it possible to price them separately and to check ratios on the right denominator.
What do you need to start an underground car park BOQ?
A shoring plan, a soil report and a hydrological opinion, structural drawings of the basement, a waterproofing specification booklet, and the consultants' drawings for fire safety, plumbing and electrical. The full preparation list is on the page BOQ preparation.
Can the excavation be estimated without a natural ground level?
Not in the bill. Without the natural ground level, the excavation item stays marked as missing, and in a cost estimate it gets a stated assumption with a source. An estimate that looks like a measurement is the costly mistake in this chapter.
Who checks the bill against the consultants' bills?
That is the work of a quantity surveyor for construction projects: to collect the consultants' bills as they are, to check that their basic numbers, such as the number of levels and the areas, match the structure, and to flag every gap as a question.
Have a project? Tell me briefly what the project is and what already exists. When the project is a good fit, I usually reply with focused questions and a written quote.
Contact me for a quote* Created with AI for marketing, for illustration only. Not reviewed by Yaakov Avital and not professional advice. Every project must be checked on its own by a professional quantity surveyor and an engineer.