Your garden shed may count against the town's carbon

Whole-life carbon assessment, timber garden shed, 2.4 m × 1.8 m, single storey, no foundation: £340.00. The line would appear on the invoice of anyone in one district in the English Midlands who wished to put a shed at the bottom of the garden after 2030, under a scheme extending the council's statutory carbon budget to outbuildings. The shed itself would cost £289.

Carbon budgets, meaning legally binding caps on total emissions over multi-year periods, exist at national level in several countries and have been adopted by many cities and local authorities as well. Building regulation is shifting from operational energy alone toward whole-life carbon, which counts the emissions embodied in materials and construction, and assessments of this kind are becoming mandatory for larger new buildings in parts of Europe. Garden buildings and garden offices grew markedly in popularity after the pandemic normalised working from home. Planning systems already regulate outbuildings through limits on size, height and siting. Carbon accounting has steadily extended into smaller categories of activity that were once left uncounted.

Under the scheme, every roofed structure in the district would be entered on a carbon ledger held by the planning department, which since the late 2020s would have been required to keep its five-year carbon budget in balance in the same way it keeps the money one. Houses, extensions and commercial units have counted for years. Sheds, greenhouses, log stores and any roofed enclosure over two square metres would be added on the grounds that a budget cannot be balanced against things that are not in it. A shed absent from the ledger would be, for planning purposes, a pile of timber that happens to be standing up.

A standard shed of pressure-treated softwood, a felt roof and galvanised hinges would carry an embodied figure of roughly 0.4 tonnes, most of it in the felt and the hinges. Applicants could reduce the figure by choosing untreated timber, which rots, or a reclaimed door, which does not fit. The assessment would be valid for the shed's design life, set at fifteen years, after which the owner would be invited to submit an end-of-life plan. "I asked what happens if the shed is still fine at sixteen," said Gareth Pollard, a retired surveyor. "They said it would still be fine. It would just be fine outside the budget."

The district's residual budget for 2030 to 2035 would be allocated first to housing, then to the secondary school rebuild, then to discretionary residential structures, a category in which sheds would compete with conservatories, hot tubs and the larger kind of trampoline. When the discretionary allocation runs out, which the council's own modelling suggests would happen in the second spring of each five-year period, new sheds would go on a waiting list. The list would be ordered by date of application, with priority for anyone who could show that the shed would displace a car.

Nadia Ferris, who applied for a 1.8-metre shed to keep two bicycles dry, would receive a letter confirming that her application had been accepted in principle and placed at 214 in the queue. The letter would note that the bicycles could be stored indoors in the interim and enclose a leaflet on hallway storage. "They asked me to estimate how many car journeys the bicycles would replace over the shed's design life," she said. "I put a number. I don't know if it was the right number. The shed is for the bikes, the bikes are for the shops, and the shops are four hundred metres away."

Households with an existing shed would find themselves holding an asset. A shed registered before the scheme would carry a legacy allocation, transferable with the property or, under a proposed amendment, surrendered to the council in exchange for a credit against a future extension. One estate agent in the district would already list "shed, registered, 0.4 t" between the number of bedrooms and the boiler. A neighbouring parish would record the first dispute over a shed standing exactly on a boundary, whose tonnage each side would decline to accept. Both households would agree that the shed was very useful and belonged to the other.

Sheds used as offices, of which the district would count several hundred since the years of home working, would be assessed as commercial floor space at the higher rate, unless the occupant could show that the alternative was a daily commute. Deirdre Mallon, the council's carbon accounting officer, would decline to call this a penalty. "The budget is the budget," she would say. "We do not keep a separate budget for things people consider small. A tonne in a shed weighs the same as a tonne in a warehouse, and we have been asked about that too."

A committee member would propose that demolishing a shed should return its tonnage to the pool, and the planning department would confirm that a shed which falls down of its own accord does so already, without paperwork. Residents would not be obliged to hasten this. Mr Pollard, whose shed dates from 2009 and leans, would say he had read the guidance twice. "The cheapest thing I can do is keep the old one standing and never mention the roof," he said. "It has a design life of fifteen years, so officially it has been gone for six. I keep the mower in it."