A week today, we will have the oddity of a Westminster set-piece that was never supposed to be. Budgets, the Queen's speech and the like can usually be plotted out years in advance. But this month's spending review – or, rather, cuts review – was not expected to be necessary. The coalition originally ventured that the retrenchments it set out in 2010 would suffice to fix the public finances within a single parliament. But as the recovery failed to materialise, five years of austerity became six years, then seven and more, and it became unavoidable for George Osborne to detail an extra year of pain for the financial year 2015-16, which straddles the next general election.

We are all used to reading about 15%, 20% or 25% cuts, so the 2.8% saving that the chancellor requires from the average department might sound easy to find. And yet there are rumbling government rows over services such as the police. For the sanguine analysis that says a couple of percentage points is nothing to worry about ignores two crucial things: first, the fact that – after a half-decade of pain – the easy cuts have all been made; second, the continuing promise to exempt big chunks of spending, most notably the huge health service, from outright cuts redoubles the pain elsewhere.

Indeed, factoring in the "ringfences" for hospitals and schools, and on realistic assumptions about the ability of Philip Hammond and Theresa May to shield squaddies and bobbies from the worst, the Institute for Fiscal Studies reckons (pdf) that other departments might be facing cuts of around 10%. That is a savage saving to have to find in any year, and especially in a year that follows such a long lean spell.

Think about what a cut on this scale would mean for an urban town hall, where the cuts have already reduced spending power by substantially more than the English average of 12.2%. Statutory duties towards orphaned young people and frail elderly people consume a huge chunk of resources, and cannot lawfully be disowned. Council back-office services have already been pared to the bone – planning and development has been cut by an average of 46%. Meanwhile, typical cultural spending has been chopped by a fifth. If you care about a library, a sports centre or even a park close to your home, it is time to be afraid.

With the Treasury's plans implying that – after the election – we will hear about detailed plans for yet another two years of similar pain, we are talking about cumulative cuts that go so far beyond what the public has been primed for that they could strain the social contract. No wonder those in the know are scrambling around for alternatives. Treasury officials slipped a telling sentence into the budget: "It would, of course, be possible to do more of this further consolidation through tax instead." No doubt we can expect a conspiracy of silence from the politicians on that point until polling day is safely out of the way; for now, the vaunted Osborne alternative will be cutting welfare – again.

He wants to build on his benefit cap for individual families by somehow capping welfare spending as a whole. The counter-argument about poverty won't interest him, but as the guardian of the economy he should be expected to give weight to the crucial role that social security payments have played in steadying demand in the slump. He might also recall the last time that spending which bounces about because of booms and busts was lumped in with the rest – in the late 80s and early 90s. The government ended up in a mess because it mistook the flattering effects of a brief burst of growth for a permanent improvement.

In the end, the pickle we're in will take taxes to fix – but don't expect anyone to tell you that next week.