Garages, outbuildings and "small" builds: the checks people skip because it doesn't feel like a proper project

Most of the projects I look at are big enough that nobody questions whether they need checking properly — a two-storey extension or a new dwelling gets an architect, a structural engineer, probably a planning consultant, as a matter of course. The projects that genuinely worry me are the small ones: a garden office, a detached garage, a modest single-storey outbuilding, priced at a fraction of a house extension and treated, as a direct result, as though it doesn't really count as a project at all.

The size of the budget doesn't change the size of the risk

A badly specified foundation, an unconsented structure, or a drainage connection that was never actually checked causes the same category of problem whether it sits under a £15,000 garden room or a £400,000 extension — an enforcement notice, a structure that needs underpinning, a bathroom that can't drain. What changes at the smaller scale isn't the risk, it's the amount of professional attention the build tends to get before anyone commits money to it, which in my experience is usually close to none.

Planning: permitted development is a specific set of limits, not a blanket exemption

"It's permitted development" gets said about garden buildings as though the phrase settles the question on its own. It doesn't — permitted development for outbuildings is a specific, fairly detailed set of limits on height, position, coverage and use (covered in full elsewhere on this site), and a building that's forward of the house, too tall for its distance from the boundary, or not genuinely incidental to the house, falls outside it regardless of how modest it looks. The assumption that "small equals permitted" is the single most common planning mistake I see on this scale of project.

Building Regulations kick in earlier than the word "outbuilding" suggests

A detached garden building under 15m² with no sleeping accommodation is commonly exempt from Building Regulations — but that exemption narrows fast: cross a 15m² or 30m² threshold, sit within 1 metre of a boundary, add a fixed combustion appliance, or put in anything for sleeping, and full Building Regulations approval is back on the table (the mechanics are set out in more detail in the foundations and services article on this site). A surprising number of "simple" builds cross one of those thresholds without anyone noticing until Building Control, a neighbour, or a future buyer's conveyancer asks the question.

Drainage: the question that gets asked after the slab is poured

A garden room with a WC, or a workshop with a sink, needs somewhere for the water to actually go, and on a modest build that question is routinely left until the building is largely finished — at which point the options narrow to a pumped connection, a small package treatment solution, or an expensive retrofit, none of which were priced into the original quote. Establishing the drainage route and the available fall is a half-hour check at the design stage and a genuinely difficult, expensive one once the floor is down.

Party wall rules don't care that the building is small

A garage or outbuilding built close to a boundary, with foundations that excavate within 3 metres of a neighbour's structure (or 6 metres, depending on depth), falls under the Party Wall Act in exactly the same way a full extension would. The scale of the building has no bearing on whether notice is required — only its position relative to the boundary and the neighbour's foundations does, and a late or missing notice is one of the most avoidable causes of a stalled start on a project this size, because nobody thought a shed warranted the paperwork.

Structural questions under a modest roof

A single-storey outbuilding carries real structural questions even though it doesn't look like it: foundation design matched to the actual ground rather than assumed adequate because the building is light, a roof structure genuinely sized for its span and loading rather than copied from a similar-looking shed, and — where the building sits against or very close to an existing structure — a proper look at how the two interact rather than treating them as unconnected. None of this needs to be expensive to get checked; it needs to actually happen before the foundation is dug.

The checks worth running before you commit to anything

• Confirm the proposal actually sits within permitted development limits for height, position and coverage — or get a Lawful Development Certificate before building if there's any doubt
• Check the Building Regulations exemption thresholds against the building's actual size, boundary distance, intended use and any fixed appliances
• Establish the drainage route and available fall before the design is finalised, if the building will have a WC or a sink
• Check the foundation's proximity to any boundary and neighbouring structure against Party Wall Act thresholds, and serve notice early if it applies
• Have the foundation design matched to the actual ground conditions, not assumed adequate because the building is small
• Check buildings insurance and, if the building will be let or lived in even informally, the main house's mortgage terms