Optimism bias uplift
Apply HM Treasury’s generic optimism bias adjustment for your project type to your own capital estimate and programme, in money and months, at the upper and lower bound, and see which bound the guidance calls the starting point at your stage.
The seamThe appraiser who wrote the estimate and the board that will read it as a forecast, with the adjustment the Green Book requires between them and usually nowhere on the page.
The tool opens on the guidance's own worked example, and prints the document's answer under the result so you can check the arithmetic before you trust it with anything. Replace the figures with your own.
The guidance's six types, plus its method for a project that mixes standard and non-standard work that cannot be physically separated.
Changes nothing in the arithmetic. It changes which bound the guidance calls the starting point, and the result says which.
The guidance applies the factor to the present value of capital cost. Any currency; the output is in the same one.
100,000,000
The estimated construction period. Leave blank to skip the duration uplift.
Non-standard building. Buildings requiring special design considerations from space constraints, complicated sites, specialist or innovative design, or unusual output specifications. The guidance names specialist hospitals, innovative prisons, high technology facilities, other unique buildings and refurbishment projects.
Optimism bias uplift
2026-10-01. projects-advisors.com/tools/optimism-bias
- Project type
- Non-standard building
- Stage
- Outline business case
- Capex estimate
- 100,000,000
- Works duration
- 28 months
Capital expenditure with the published uplift
104,000,000 to 151,000,000
HM Treasury's generic optimism bias adjustment for capital expenditure is +4% at the lower bound and +51% at the upper, the published range for non-standard building projects. The upper bound is the average overrun the underlying study found at outline business case; the lower is the level to aim for by contract award. On duration, +2% to +39%: 28.6 to 38.9 months against your 28.
| Bound | Published | On this estimate | On this duration |
|---|---|---|---|
| Lower bound, the target by contract award | +4% / +2% | 104,000,000 | 28.6 months |
| Upper bound, the outline business case average | +51% / +39% | 151,000,000 | 38.9 months |
At outline business case
The stage the upper bound was measured at (3.3). The guidance says to start there (3.11) and to reduce it only for contributory factors whose mitigation has been independently verified (3.17).
What a reduction would have to be evidenced against
The guidance reduces the upper bound in proportion to how far each of 21 contributory factors has been mitigated, and says that clear and tangible evidence of the mitigation must be observed and should be independently verified before any reduction is made (3.17), naming the Gateway Review as such a procedure. This page does not ask you to rate your own mitigation and return a lower number; that would be your belief with the guidance's arithmetic attached. What it can show is the list a reviewer will work down.
- Procurement
- Complexity of contract structure; Late contractor involvement in design; Poor contractor capabilities; Government guidelines; Dispute and claims occurred; Information management.
- Project specific
- Design complexity; Degree of innovation; Environmental impact.
- Client specific
- Inadequacy of the business case; Large number of stakeholders; Funding availability; Project management team; Poor project intelligence.
- Environment
- Public relations; Site characteristics; Permits, consents, approvals.
- External influences
- Political; Economic; Legislation and regulations; Technology.
What the Green Book 2026 says this table is for
Paragraph 6.80: the size of the adjustment should be informed by the originating organisation's own evidence of its forecast errors. Paragraph 6.81: only if a public body has no evidence of its own historical optimism bias may it use these generic values. Paragraph 6.83: contingency is the remaining optimism bias adjustment plus the value of risks not prevented. So the figures above are the Green Book's fallback, for an owner with no record of their own.
HM Treasury, Supplementary Green Book Guidance: Optimism Bias, Table 1, read in full 5 September 2026. Published on gov.uk on 21 April 2013 and not revised since; the figures rest on one study, Mott MacDonald (2002), of large UK public procurement. The guidance calls them indicative starting values and says the upper bound is not the highest optimism bias that can occur nor the lower the lowest that can be achieved (footnote 2). It gives no values for operating expenditure outside outsourcing, and none for benefits (4.1). Applied to a project outside the UK public sector these are a borrowed benchmark, and should be quoted as one.
This tool and the estimate range check answer different questions from different sources. That one applies an accuracy band to a declared estimate class and has no free range for hospitals, transport or one-of-a-kind buildings, because AACE's free matrices exclude them. This one applies a bias adjustment to a project type, and general hospitals, specialist hospitals, roads and rail each have a row.
Computed from
- HM Treasury, Supplementary Green Book Guidance, Optimism Bias (published on gov.uk 21 April 2013, no later revision; evidence base 2002; read 2026-09-05)https://assets.publishing.service.gov.uk/media/5a74dae740f0b65f61322c72/Optimism_bias.pdf
- HM Treasury, The Green Book, 2026 edition (2026 edition, published 5 February 2026; read 2026-09-05)https://assets.publishing.service.gov.uk/media/698dbcd17da91680ad7f4308/The_Green_Book_2026.pdf
Table 1 in full: Standard buildings 2% to 24%; Non-standard buildings 4% to 51%; Standard civil engineering 3% to 44%; Non-standard civil engineering 6% to 66%; Equipment and development 10% to 200%; Outsourcing 0% to 41%.