6m-deep rear extensions are permitted development, which means you don't need planning permission. But you still need to apply for prior approval. If that is refused, you should appeal.
Most houses in the UK can be extended without needing full planning permission - the extensions are permitted development. For example, terraced and semi-detached houses can build a rear extension to a depth of 3m into the garden. Detached houses can be extended to 4m.
In 2013, the government decided that householders should be allowed to extend up to 6m (on a terraced/semi-detached house) or 8m (on a detached house). They are known as larger home extensions. They wouldn't need to apply for planning permission but, unlike with standard permitted development rights, they would need to apply for prior approval.
Applying for prior approval is much easier than applying for full planning permission. You fill out a quick form, pay a small fee (of £96) and provide a block plan showing the location of the extension. You don't need to provide the detailed plans that would be expected for a planning application.
On receiving the application, the council consults your immediate neighbours (the properties with which you share a boundary - normally the neighbours to either side and to the rear). If the neighbours do not object, prior approval is not required, your proposal is permitted development and the council cannot refuse your application.
However, if neighbours do object, the council gains the power to assess the proposal. The only issue they can look at is whether the extension harms your neighbours' living conditions. The council cannot look at the design and appearance of the extension for example (or whether it harms the great crested newt, or a nearby tree).
The council will assess the impact on all neighbours, not just the neighbour who objected. The case officer will assess whether the extension is likely to lead to a loss of light, outlook or privacy, and whether it will represent an overbearing or unneighbourly physical presence.
The problem is that, where a neighbour complains, many councils will refuse the application. Case officers tend to think that extensions 6m deep are just too large. They refuse permission almost automatically, as a reflex. They don't necessarily look at the site and your proposal closely, to make a balanced and objective judgement of the likely impact.
It is important to be willing to appeal. We appeal dozens of larger home extension/prior approval refusals every year and have a high success rate. The high success rate is because, in our opinion, councils are refusing permission without properly assessing individual cases.
In general, a single-storey rear extension will not cause significant harm to neighbours. This is especially the case where neighbours have their own extensions but even in the absence of extensions at neighbouring properties, a ground floor extension is low in height and is not really tall enough to have a materially overbearing impact or to cast a long shadow.
If you have been refused prior approval, you should consider appealing. Our very experienced, chartered planning consultants know exactly how to make the strongest possible case (usually quoting other appeals in your area that have been successful under similar circumstances). We charge a single fixed fee of only £399 - find out more on our appeals page.
If you are not sure whether an appeal would be successful or are wondering whether it might be better to amend the plans and resubmit, you should try our case review service. For a fee of £99, our planners will look carefully at your proposal and give you bespoke, independent advice. More on that service can be found on the case review page.
It is important to remember that councils do not always reach the right decisions and, if you are refused planning permission (or prior approval), you should not give up!