The Budget will be presented on Wednesday 28 October 2026, alongside a fresh forecast from the Office for Budget Responsibility (OBR). This article explains the idea that dominates the run-up to it, fiscal headroom, and why the forecast matters as much as the speech.
When is the Budget, and what arrives with it?
According to GOV.UK, the Chancellor of the Exchequer will present the Budget 2026 to Parliament on Wednesday 28 October 2026. In a letter to the Treasury Select Committee, the Chancellor said the OBR had been asked to prepare an economic and fiscal forecast for publication on 28 October 2026, and that the government “will abide by the fiscal rules, ensuring we retain a buffer to protect us against uncertainty”.
That sentence contains the two ideas this article unpacks: fiscal rules, and the buffer that sits between the forecast and those rules. The OBR is independent of the Treasury, which is why the forecast is published by a separate body rather than written by the department that sets tax and spending.
What are the fiscal rules?
The government has two headline rules. According to the Budget 2025 document on GOV.UK, the stability rule relates to the current budget, which is day-to-day spending set against revenue, and the document said the OBR forecast the current budget would move into surplus in 2028-29, “meeting the stability rule a year early”. The investment rule relates to net financial debt, which the same document said would fall as a share of GDP in 2029-30 and 2030-31, “meeting the investment rule in 2029-30”.
In plain terms, one rule asks whether the government is paying for its everyday running costs, and the other asks whether the debt burden is shrinking relative to the size of the economy. Borrowing to invest is treated differently from borrowing to cover routine spending.
The year in which a rule is tested matters. The March 2026 headroom figures below are measured in 2029-30, and a later forecast can test the rules against a different year, which alone can change the size of the cushion.
What does fiscal headroom actually mean?
Headroom is the margin by which the OBR’s central forecast meets a rule. If the forecast shows the current budget in surplus by £20 billion in the target year, the rule is met with £20 billion to spare. If it showed a deficit, the rule would be missed.
The Treasury’s own language is slightly different. Budget 2025 spoke of a “buffer” of £21.7 billion against the stability rule and of a “margin” for the investment rule, which it said was met “by a margin of £24.4 billion”. The words differ, but the logic is the same: headroom is the cushion a Chancellor can spend on tax cuts or extra spending, or keep as protection, without breaching a rule on the OBR’s numbers.
How big was the cushion at the last forecast?
The most recent official reading came with the Spring Forecast in March 2026. According to the Chancellor’s Spring Forecast 2026 speech on GOV.UK, headroom against the stability rule in 2029-30 rose from £21.7 billion to £23.6 billion, and headroom against the investment rule was £27.1 billion.
| Measure (2029-30 target year) | Autumn 2025 | March 2026 |
|---|---|---|
| Headroom against the stability rule | £21.7 billion | £23.6 billion |
| Headroom against the investment rule | £24.4 billion | £27.1 billion |
The difference for the stability rule is £23.6 billion minus £21.7 billion, or £1.9 billion. For the investment rule it is £27.1 billion minus £24.4 billion, or £2.7 billion. These are changes in a forecast margin, not new money in the Treasury’s account.
Note also that the March 2026 figures are now several months old. Everything that has happened since, from borrowing outturns to the cost of servicing debt, is what the October forecast will fold in.
Why can headroom change before the Chancellor says a word?
Headroom is a residual: a forecast outcome minus a fixed line. Any change to the forecast moves it, and the government does not have to do anything for that to happen. Four things typically feed in:
- Borrowing so far. Actual monthly outturns reset the starting point of the forecast.
- Debt interest. Interest costs depend on gilt yields and, for index-linked gilts, on inflation.
- The economy. Weaker growth tends to reduce tax receipts, while higher growth tends to lift them.
- Methodology. The OBR can change how it models particular taxes or spending lines.
The last two points are why the same Chancellor, with unchanged policies, can find the cushion larger or smaller from one forecast to the next. For more on how the underlying growth data is built and later revised, see our guide to how UK GDP is measured and revised.
What do the latest public finance figures say?
The Office for National Statistics (ONS) published its September 2026 public sector finances on 21 October 2026. It reported public sector net borrowing of £24.5 billion in September, £1.9 billion above the OBR forecast of £22.6 billion. Over the financial year to September 2026, borrowing was £97.8 billion, which was £4.4 billion lower than a year earlier but £6.0 billion above the OBR forecast of £91.8 billion. Those OBR comparisons use the March 2026 forecast.
Public sector net debt at the end of September 2026 was provisionally £3,005.0 billion, and the ONS said this was the first time it had exceeded £3 trillion. As a share of GDP it was 94.0%, which was 0.7 percentage points lower than a year earlier and 1.2 percentage points below the OBR forecast. The ONS noted the figure may be revised.
Central government debt interest payable in September 2026 was £10.9 billion, £2.0 billion above the OBR forecast, and included £4.0 billion from the capital uplift on index-linked gilts. For a fuller explanation of why debt can rise in pounds while falling against GDP, read our guide to UK deficit and debt and what the public finances show.
How might a small change swallow the cushion? A worked example
The arithmetic below is illustrative and not a forecast. Suppose the stability-rule headroom in the target year were the March 2026 figure of £23.6 billion. Now suppose the new forecast adds £10 billion to expected annual debt interest in that year, and nothing else changes.
- Starting headroom: £23.6 billion
- Extra debt interest: £10.0 billion
- Remaining headroom: £23.6 billion minus £10.0 billion, which is £13.6 billion
If the Chancellor then wanted to announce measures costing £15 billion a year in the target year, the rule would be missed by £1.4 billion (£15.0 billion minus £13.6 billion) unless other measures offset the gap. The example shows why a headline figure for headroom is not a fixed budget: it is the end-product of a long chain of assumptions.
Why do interest rates and inflation matter to headroom?
Two of the biggest influences come from outside the Treasury. Gilt yields set the cost of new government borrowing, and they respond to expectations about the Bank of England’s decisions. Our explainer on why Bank of England rate decisions ripple through the economy covers that chain from policy rate to household borrowing.
Inflation matters twice. It affects tax receipts and spending over time, and it feeds directly into the cost of index-linked gilts, which is why the ONS singled out the £4.0 billion capital uplift in September’s debt interest. To see how the headline measure is constructed, see what inflation figures really tell us.
How should readers treat Budget-week headlines?
A few habits help when the numbers start to arrive on 28 October:
- Check the target year. A rule met in one year can still look uncomfortable in the years either side.
- Separate forecasts from decisions. The OBR publishes the central forecast and assesses the policies the Chancellor announces; the political choices are the Chancellor’s.
- Read headroom as a range. A cushion of a few tens of billions is small relative to total spending, so modest forecast changes can shift it noticeably.
- Look past the headline. Individual tax and spending measures often phase in over several years, so the first-year cost can differ from the target-year cost.
What the numbers on 28 October will and will not settle
The forecast will show how far the public finances have moved since March, and whether the cushion against each rule is larger, smaller or about the same. It will not settle how a given household is affected, because that depends on measures that are still unannounced at the time of writing. The next ONS public sector finances release is due on 20 November 2026, which will give the first outturn data after the Budget.
This article provides general information, not personalised financial advice. Rules, forecasts and rates change.
Sources: GOV.UK; Office for National Statistics
