Every kitchen-table extension conversation reaches the same confident sentence: “you can do forty square metres without permission.” The confident sentence is a headline on a contract nobody read: the exemption is real, and it’s wrapped in conditions that remove it entirely when any one is breached.
The Wrapping Around the Number
The exemption class for extending a house comes with the full wrap: the floor-area threshold counted cumulatively — against every previous extension to the house, including the one the last owner built and never mentioned, which is how “our modest twenty square metres” becomes over-threshold before a block is laid; height and level rules — ground-floor and first-floor elements treated differently, with distance-from-boundary conditions attaching to upper-level works and their windows; the rear-only orientation of the class — extensions to the front are a different conversation; the remaining garden — open private space that must survive the works; and the elements that were never in the class at all — balconies and roof terraces change the analysis entirely. The governing trap is totality: breach one condition and the exemption is lost for the whole works — not the offending metre — leaving the entire extension unauthorised, with everything that means for enforcement and, more predictably, for the day you sell.
The Method: History First, Certainty Where It’s Cheap
The order of operations for any extension: the property’s cumulative history first — what’s been added since the house was built, by anyone, measured, because your allowance is what remains of the threshold, not the threshold; the design against the actual conditions — heights, boundaries, windows, garden area, checked at drawing stage where changes cost nothing; and certainty bought where any edge exists — the Section 5 declaration converting “the architect thinks it’s exempt” into the authority’s formal determination, cheap insurance against the expensive direction: retro-fitting compliance to a built structure. Buying a house with extensions? The same method inverts into due diligence — the works history and its paperwork requisitioned before contracts, per the conveyancing guide. And where the design genuinely exceeds the exemption’s world: the planning application is not a defeat — it’s the route that produces a permission you can prove forever, which the sale will one day thank you for.
The sentence to replace the folklore: not “you can do forty square metres” but “the house may have exemption capacity remaining — measured against its full history and the conditions, on this design.” Less quotable at the kitchen table; far cheaper at the boundary wall.
An extension at drawing stage — or one already built and queried? The history and the conditions, checked: 01 5827148.