Planning an extension within the New Forest National Park can be notoriously tricky. So what are the rules, and where do they apply?

Based in Salisbury, Esdon Architecture is just a few miles from the edge of the New Forest, and as a practice we’ve been fortunate to work on a number of renovations and extensions within the National Park over the years. While every project is different, there are some key principles that homeowners should understand before getting started.

The 30% Rule

As a general rule (and, as always, there are exceptions), extensions within the New Forest National Park (NFNP) are limited to 30% of the existing habitable floor space.

Crucially, “existing” doesn’t necessarily mean what’s there today. Habitable floor space is assessed as:

• the size of the property as it existed on 1 July 1982, or
• if the property was built after that date, as originally constructed.

This can make designing an extension feel a bit like a game of Tetris. When space is at a premium, every square metre matters, and careful planning is essential to make the most of the allowance.

Small Dwellings

If a property has an existing floor area of less than 80m², it is classed as a 'small dwelling'. In these cases, extensions are restricted so that the total floor space cannot exceed 100m², regardless of the 30% calculation.

Village Exceptions

An important exception to the 30% rule applies to properties located within one of the four main New Forest villages:

• Ashurst
• Brockenhurst
• Lyndhurst
• Sway

In these locations, larger extensions may be permitted, subject to normal planning considerations such as design, scale and impact on neighbouring properties.

Further Guidance

More detailed guidance — including additional exemptions — can be found in the New Forest National Park Authority’s Planning Information Leaflet: Domestic Extensions and Replacement Dwellings (January 2022), available via the NFNPA website.

01160-Planning-info-leaflet-domestic-extensions-and-replacement-dwellings-REVjan2022-TO-WEB.pdf

A Recent Project

This summer, we were delighted to secure planning permission for clients who had purchased a traditional but uninhabitable cottage just within the National Park boundary. The approved scheme creates new upstairs bedrooms, a family bathroom and ensuite, alongside a ground-floor extension to form a modern family living space — all while carefully working within the planning constraints of the NFNP. Works are now underway on site, with completion scheduled for summer 2026.

Planning an extension in the New Forest?

Get in touch with the team at Esdon Architecture and we will be happy to chat through your proposals.