Will your project need planning permission?

Often not. Most modest single-storey rear extensions fall under permitted development if they stay within the national size limits. You will need a planning application if the house is listed, if permitted development rights have been removed by an Article 4 direction, if the extension goes to the side in a conservation area, or if it exceeds the GPDO 2015 limits. The checker on this page applies those rules to your address.

Under permitted development a single-storey rear extension can project up to 3 metres beyond the original rear wall on a semi or terrace, or 4 metres on a detached house, with a maximum height of 4 metres. Larger extensions, up to 6 metres attached or 8 metres detached, can go through the Larger Home Extension prior approval process instead. A two-storey rear extension is capped at 3 metres deep and must finish at least 7 metres from the rear boundary.

Sometimes, but the routes are narrow. National policy treats new buildings in the green belt as inappropriate except in limited cases: infill within a village, replacement of an existing dwelling that is not materially larger, limited affordable housing, and redevelopment of previously developed land. Much of the St Albans district and the surrounding Hertfordshire countryside is green belt, which is why we assess planning context before any design work starts. A plot in the green belt is not a dead end, it just needs the case built properly.

Yes. Demolition and replacement of a dwelling needs a full planning application. In the green belt, the replacement must not be materially larger than the house it replaces, though what counts as materially larger turns on the council's policy and the site. Knock down and rebuild is often the strongest route to a genuinely bespoke home on an existing plot, and it is a core part of what we design and build.

Usually not, if the added roof volume stays under 40 cubic metres on a terrace or 50 cubic metres on a semi or detached house, nothing extends beyond the front roof plane, and the ridge is not raised. Loft conversions are not permitted development in conservation areas or on flats, and side-facing windows must be obscure glazed. Where it qualifies, a Lawful Development Certificate is worth having for £274.

A council rule that removes permitted development rights in a defined area, so work that would normally be automatic needs a planning application instead. Councils use them mainly to protect conservation areas. Several Hertfordshire authorities, including St Albans, have Article 4 directions in force, which is why the checker tests for them at your address rather than assuming.

A householder application in England costs £548 from 1 April 2026, a full application for a single new dwelling £610, a Larger Home Extension prior approval £249, and a householder Lawful Development Certificate £274. Fees rise each April with CPI. Design fees for drawings, the planning submission and structural engineering are separate, and we define those clearly during the Feasibility Study.

Householder and other minor applications have an eight-week statutory target, major applications thirteen weeks, and Larger Home Extension prior approvals are decided within 42 days or deemed approved. The clock starts at validation, not submission, and a well-prepared application validates first time. That preparation is most of the battle.

Both are legally planning permission. A planning application is decided by your council case by case. Permitted development is a national grant of permission issued in advance by the GPDO 2015, covering common projects like modest extensions, lofts and outbuildings, subject to strict size and siting limits. If your project fits inside those limits, no application is needed, though a Lawful Development Certificate formally confirms it.

It applies the national rules faithfully and checks live designation data where the national datasets cover your address, and it tells you plainly when an answer depends on a measurement or the council's judgement. It is guidance, not a legal determination, and it never replaces confirmation from your local planning authority. Where your answers leave genuine doubt, the report says so and recommends the check that settles it.