Do you need planning permission? Permitted development explained

Not every extension or loft conversion needs a full planning application, and knowing where the line sits under permitted development can save both time and the £548 cost of a householder planning application, provided the project genuinely fits within the rules.

Permitted development, commonly abbreviated PD, is a set of national rights under the General Permitted Development Order that let certain categories of building work go ahead without a formal planning application, provided they stay within specific size and design limits. For a single-storey rear extension, standard PD allows up to 3 metres depth from the original rear wall for a semi-detached or terraced house, or 4 metres for a detached house. A larger extension, up to 6 metres for a semi-detached or terraced house or 8 metres for a detached one, can still avoid a full application via the "larger home extension" prior approval route, which requires notifying neighbours through the council before work starts and costs £249, considerably less than a full application, though the council can still object if a neighbour raises a valid concern.

Height limits apply on top of the depth rules and catch some homeowners out. Maximum ridge height under PD is 4 metres, eaves height must not exceed the height of the existing eaves, and for any part of an extension within 2 metres of a boundary, maximum eaves height drops to 3 metres regardless of the rest of the building. A design that fits comfortably within the depth limit can still fall outside PD if the roof pitch or eaves height doesn't meet these separate conditions, so it's worth checking all three measurements together rather than just the headline depth figure.

Loft conversions have their own separate PD allowance, commonly cited as an additional roof volume of 40 cubic metres for a terraced house or 50 cubic metres for a detached or semi-detached house, alongside conditions requiring materials to match the existing house and any side-facing windows to be obscure-glazed and non-opening below 1.7m. Many rooflight and hip-to-gable conversions fit comfortably within this allowance; a large dormer or mansard conversion is more likely to exceed it, in which case a full planning application becomes necessary regardless of the roof type.

Outbuildings, garden rooms, garages and similar standalone structures have their own set of PD rules too, generally allowing single-storey buildings within certain height limits provided they don't cover more than 50% of the garden area, though the exact height and siting rules vary depending on how close the structure sits to a boundary and whether it's within 2 metres of one. It's worth checking these limits specifically for outbuildings rather than assuming the extension rules apply equally, since the two sets of PD rights are calculated differently.

PD rights don't apply everywhere or to everyone. They're removed entirely for listed buildings, and conservation areas, National Parks and Areas of Outstanding Natural Beauty commonly carry tighter restrictions or, in some cases, an "Article 4 direction" that removes specific PD rights for a defined area regardless of the property type. Flats and maisonettes don't benefit from the same householder PD rights as houses. Checking with the local planning authority, or reviewing the property's planning history and any Article 4 directions before assuming a project qualifies, is worth doing before finalising a design around PD limits.

Even when a project clearly fits within PD, a Lawful Development Certificate is worth considering, at a cost of £274. It doesn't grant fresh permission, since the work is already lawful without it, but it formally confirms that status in writing, which matters most when selling: a buyer's solicitor will commonly ask for evidence that any extension or loft conversion was carried out lawfully, and a certificate answers that question cleanly rather than leaving it to be argued after the fact. It also protects against PD rules changing in future, since the certificate fixes the legal position as it stood when the work was done, regardless of what the rules later become.

If a full householder planning application does turn out to be necessary, budgeting for it properly means more than just the £548 fee. There's a Planning Portal service charge of £75.83 plus VAT on top for an online submission, and if drawings haven't already been produced for a PD-based design, professional fees for planning drawings apply in addition to the application fee itself. A decision is generally expected within eight weeks of a valid application being registered, though that can extend for a more complex proposal or one that attracts objections requiring further consideration, so it's worth building a realistic time buffer into a project schedule rather than assuming a decision will land exactly on the statutory deadline.

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