Summary
Permitted Development (PD) allows certain types of work to be carried out without planning permission, but only when strict national rules are met in full. PD is not flexible and it is not a judgement call. If a proposal breaches even one rule, planning permission is required. The biggest problems we see arise where homeowners assume PD applies, particularly with wrap-around extensions, dormers, and properties in conservation areas.
Permitted Development is often presented as the easy option
Homeowners are told it’s a way to extend or alter their home without the time, cost, or uncertainty of a planning application. In some cases, that is true. But in practice, PD only works smoothly when the rules are understood properly and applied conservatively. The issue is not that PD is unclear. The issue is that it is commonly simplified, stripped of context, and treated as something you can stretch. It can’t. This article explains what PD actually is, the rules that apply, and the areas where we see homeowners get caught out most often — based on real projects, not theory.
Permitted Development rights are granted through national legislation. They allow specific forms of development to proceed without applying for planning permission, provided every relevant condition is met. PD is not discretionary. Councils do not assess design quality, neighbour impact, or whether something “seems reasonable”. Either a proposal complies with the rules or it doesn’t. There is no negotiation and no balancing exercise. This is the key difference between PD and a planning application, and it is why PD should be treated as a technical route, not a relaxed one.
Permitted Development rights apply primarily to houses. Flats and maisonettes do not benefit from the same rights, regardless of size or location. Even where a property is a house, PD rights may not apply. They can be removed by conditions attached to earlier planning permissions, restricted by conservation area controls, or limited by estate-wide planning decisions. PD is also cumulative. What has already been built on the property matters. The allowance does not reset when a house is sold, and previous extensions count whether they were built recently or decades ago.
Across all types of PD, there are consistent principles that must be satisfied. Development must remain within defined size and height limits. It must not result in a disproportionate enlargement of the original house. It must respect boundaries, roof form, and the overall envelope allowed by the legislation. Materials are generally expected to be similar in appearance to the existing house, and PD does not override Building Regulations, party wall legislation, or private legal restrictions.
If any one rule is breached, PD falls away and planning permission is required.
Porches are one of the simplest forms of PD, but the rules are still exact. A porch can usually be built under PD where the external footprint does not exceed three square metres, the overall height does not exceed three metres, and no part of the porch is within two metres of a boundary or the highway. The original front door must remain in place. Porches often fall outside PD where houses sit close to pavements or where designs become too enclosed. Removing the front door or allowing the porch to read as an extension rather than a modest addition is a common mistake.
Rear extensions are one of the most common PD projects and also one of the most misunderstood. Under standard PD rules, a detached house can extend up to four metres from the original rear wall, while a semi-detached or terraced house can extend up to three metres. The extension must not exceed four metres in overall height, and where it is close to a boundary, the eaves height must not exceed three metres. The extension must not cover more than fifty percent of the original garden area, taking into account all existing extensions and outbuildings. Measurements are taken from the original house as it stood in 1948 or when first built. Where homeowners run into trouble is not usually the rear extension in isolation — it is what happens when rear and side extensions are combined.
One of the most common misconceptions we see is the belief that a wrap-around extension can be built under PD by combining a rear extension and a side extension. This is incorrect. Permitted Development does not allow rear and side extensions to be combined into a single continuous form. Once the extensions are physically linked, the resulting scale, mass, and impact go beyond what PD is intended to allow. At that point, a full planning assessment is required. This catches homeowners out repeatedly. They design a rear extension that complies, a side extension that appears compliant on its own, and assume combining the two is acceptable. It isn’t. The moment they become one form, PD no longer applies.
Side extensions under PD are deliberately restricted. Only single-storey side extensions can qualify. The height must not exceed four metres, and where the extension is close to a boundary, the eaves height must not exceed three metres. Critically, the width of the extension must not exceed half the width of the original house. Two-storey side extensions are never permitted development and always require planning permission. Side extensions are also visually sensitive, particularly on corner plots, which is why the rules are tighter and less forgiving.
Dormers are where we see the most confusion and the most costly mistakes. PD allows certain roof extensions, but only within strict volume limits. Terraced houses are limited to forty cubic metres of additional roof volume. Detached and semi-detached houses are limited to fifty cubic metres. This volume includes all roof additions, not just the dormer itself. Homeowners frequently misunderstand how volume is calculated, often measuring only the visible dormer rather than the full roof alteration. This leads to schemes that exceed PD limits without realising it. Dormers must be set back from the eaves, must not extend above the highest part of the roof, and must not be located on the principal elevation facing the highway. Front-facing dormers are not permitted development.
Conservation areas are one of the biggest PD traps for homeowners. In many conservation areas, permitted development rights are reduced or removed altogether. Works that would be perfectly acceptable under PD elsewhere may require planning permission in these locations, even where they appear modest. We see this repeatedly. Homeowners rely on generic PD guidance without realising their property sits within a conservation area, only to discover later that the rules they were relying on simply do not apply. This is why location must always be confirmed before assuming PD.
Our professional view is clear: relying on PD without confirmation is riskier than most homeowners are led to believe.
If you build under PD without formal confirmation and a neighbour complains or the council investigates, you may be required to prove that the works are lawful. That burden sits with the homeowner.
This is why Lawful Development Certificates exist. While not mandatory in every case, they provide written confirmation that the council agrees the works are permitted. For many homeowners, that certainty is worth far more than the small upfront cost.
At Studio FRI, we do not treat PD as something to assume. We check whether PD rights still exist, assess the proposal against the rules in full, and identify where homeowners are close to thresholds or exposed to risk. Where appropriate, we advise on Lawful Development Certificates so there is certainty before work begins. This approach avoids enforcement issues, redesigns, and the stress that comes from finding out too late that PD did not apply. If you are considering a porch, rear extension, side extension, or dormer and are unsure whether permitted development genuinely applies, that uncertainty is worth resolving early. A short conversation can usually confirm whether PD applies, whether formal confirmation is sensible, or whether planning permission is required. You are welcome to book a call and talk it through.
Studio FRI is a contemporary architectural and planning practice based in Preston, working across Lancashire and the wider North West. We specialise in refined residential architecture and strategic planning, delivering calm, considered design solutions shaped by insight, proportion and purpose.
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