Car dealer preparation costs: keep the estimate, check the actual

Keep the preparation estimate you used when buying the car, then record the actual costs against the same stock record. Do not replace the original estimate with the final bill. You need both to explain why the expected contribution changed and whether the next buying allowance needs adjusting.

For a small dealership, a few extra columns in the stock sheet can do this. The important part is keeping the buying decision separate from what you learn afterwards.

What I built last week

On 2 October 2026, I added and tested saved buying assessments in a dealer management system I’m developing. Each assessment keeps the assumptions entered at the time. Saving a later assessment leaves the earlier one available, so a changed preparation allowance does not erase the original figure.

The local checks confirmed that saved assessments remained available after a restart. Missing preparation costs also stayed unknown instead of quietly becoming £0. These were local tests with fictional records. The cost comparison below shows how to use that retained estimate when the actual bills arrive.

Keep the buying estimate intact

When you assess a car, save the preparation allowance with its date and basis. Link the workshop quote or explain how you arrived at the allowance. That basis matters when you later compare it with a bill.

If you discover more work before buying, save a revised assessment and reconsider the bid. Retain the earlier version. If you have already bought the car, update the current expected result without changing the buying record.

Figure 1 shows the distinction. The original £600 stays visible even after the actual preparation total reaches £950. Those figures are an illustrative example, not a client’s costs.

Match the bill to the car and the work

Record actual preparation costs against the same vehicle identifier used in the assessment. Link the invoice or job reference so someone else can check the total. If one supplier bill covers several cars, allocate its lines to the relevant stock records rather than putting the whole bill against one car.

Compare equivalent work. A £600 allowance for mechanical preparation cannot fairly be compared with £950 that also includes transport previously budgeted elsewhere. Use the same VAT basis for the estimate and the actual cost: exclude VAT you can recover, and include VAT you cannot recover. Do not compare a net estimate with a gross invoice total.

Keep work still awaiting a bill visible. A blank actual-cost field does not establish that the job was free or that the vehicle’s preparation cost is complete. A supplier credit should reduce the relevant cost once checked; it should not become an unrelated adjustment elsewhere in the stock sheet.

Worked example: a £350 preparation overrun

Illustrative figures only. A dealer allowed £600 for preparation. The checked invoices for that same work total £950, with no further preparation bills expected. Both figures exclude any recoverable VAT. The assumed selling price and every other cost remain unchanged in this example.

Scroll across the table to read every column.

The cost variance is actual preparation cost minus the original estimate. A positive result means the work cost more than allowed.

Preparation cost variance · GBP

$$V=A-E$$(1)

£950 − £600 = £350

Actual cost − original estimate = cost variance

Here, A is the actual cost (£950), E is the saved estimate (£600), and V is the £350 overrun. If actual cost were below the estimate, the result would be negative.

Suppose the original assessment showed £2,200 expected contribution after the vehicle’s purchase cost, all entered vehicle-specific allowances and the applicable sales VAT. Replacing the £600 preparation allowance with £950 reduces that expectation to £1,850: £2,200 minus £350. Contribution here means what remains towards the dealership’s overheads and profit; it is not the business’s final net profit.

Subtract the extra £350 once. Taking the whole £950 off the £2,200 would count the original £600 twice. Until the car is sold and the remaining costs are confirmed, £1,850 is still an expected result.

Preparation costs and margin-scheme VAT are different calculations

A repair bill affects the commercial result on the car. It does not increase the vehicle’s purchase price for the VAT margin scheme. HMRC excludes repairs and refurbishment from that purchase-price calculation. Keep the commercial cost comparison separate from the scheme calculation. HMRC: working out the margin on second-hand vehicles.

In the example, only preparation cost changes. Where the vehicle qualifies for the margin scheme and the relevant purchase and selling prices stay the same, the extra repair cost does not reduce the margin used to calculate sales VAT. The £350 contribution reduction must not be quietly offset by an invented reduction in that VAT figure.

What makes the comparison useful

The preparation comparison is complete when the relevant work is finished, its costs and credits have been checked, and any difference from the estimate has an explanation. The overall vehicle result can still be open. Keep that distinction visible when reviewing unsold stock.

Ask what caused the overrun before changing your buying allowance. Damage discovered during preparation calls for a different response from work you left out of the original allowance. A single car does not establish a new average preparation cost.

You can keep this comparison in the existing stock sheet. A system becomes useful when several people update the same vehicles and the original buying assumptions are otherwise being lost. The separate dealership handover guide covers the checks before a sold car leaves the business.

If your stock records cannot explain the difference between the buying estimate and the eventual result, In Good Order’s finance process review can help establish where the information is being lost.

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Guidance checked on 5 October 2026. The work example concerns a tested local system; all worked-example amounts are fictional.