The New Forest National Park Authority – The 30% Rule

The Planning process can be a daunting subject and is often conceived as a worrying stage of any property development.   However, once you understand the principles of Planning Policy it becomes much clearer and understandable, less scary.   One Planning Policy that can be confusing and frustrating to many is The New Forest National Park Authority’s 30% rule.   So this blog is intended to provide some understanding of this one Planning Policy.

The first beneficial caveat to point out is that this rule does not apply within the settlement boundary of Defined Villages.   The Defined Villages are: Ashurst, Brockenhurst, Lyndhurst & Sway. Search for Defined Villages in New Forest National Park Authority to find a PDF with a set of maps showing these boundary lines and check if your property is inside or outside of the magic line.

The 30% rule limits residential development so that any dwelling can only be expanded by 30% of its original floor area. The original floor area is given a datum date that it is measured from, which is 01 July 1982 and you can add 30% of the area that existed at that time. If the building is newer than this date it will be measured from its original floor area from when it was built.

The floor area is measured across all floors, not just the footprint of the building.

Floor area is measured internally and as a gross measurement, meaning that it includes all internal walls, chimneys, cupboards and such. So excludes the thickness of the external walls. You should also measure across staircases and any double-height voids as these could be infilled without local authority or public knowledge. This initial measurement must specifically exclude any attached garages, outbuildings or conservatories as, at this stage, they are considered non-habitable. This calculated floor area becomes your original 100% floor area and where you can determine your 30% from. 

You then measure your 30% for your proposed development. This does include any extension over any number of floor levels and including basements, loft conversions or conversions of any attached garage, outbuilding or conservatory into a habitable space.

If you retain any attached garages, outbuildings or conservatories on the property, they too must form part of your 30% as they could easily be converted into a habitable space without public or local authority knowledge. Again this is measured as a gross internal like before. You can demolish any of those attached garages, outbuildings or conservatories as part of the development to allow for more area to be added as part of the extension area, otherwise, they are counting against the potential development area.

If the property is a Listed Building, separate Listed Building Consent will be required in addition to ensuring that the original heritage of the building is maintained unharmed. You might find that the garage, outbuilding or even conservatory is part of the protected building and therefore unable to have it removed. Check with your local Conservation Officer to find out more.

You can propose separate, detached outbuildings or garages to the plot, which is beneficial if you are removing an attached one as part of the development. Outbuildings and garages are covered by a different Planning Policy, which I can discuss in another blog. Outbuildings (or structures that are detached from the main house) are not restricted by the 30% rule and are therefore not measured against the area of the house.   They do need to be in proportion to the size of the plot.

Permitted Development is not directly controlled by the 30% rule either. However, New Forest National Park Authority has this covered. If you seek Planning Permission for an extension under the 30% rule that Approval Notice will be conditioned to remove any further Permitted Development Rights, subject to a further Planning Application. If you had maximised your 30% through an extension the local authority will not approve any other additional floor area attached to the house and they are very strict with this.   Depending upon the size of the original dwelling, it is possible that an extension proposed under Permitted Development, in lieu of the 30% rule, may result in greater than 30%. 

If an extension under Permitted Development is constructed you will thereafter no longer be able to apply for more extensions as your property will be seen to have used or even exceeded the allowable 30%.

The New Forest National Park Authority are very strict with this policy and will refuse even 0.1% over the 30%.   Using a CAD programme and a professional measured survey will provide you with the evidence to accurately demonstrate the measurement of the original floor area. We always provide a separate drawing that clearly demonstrates the original and proposed areas for the local authority in this instance.   If you have any questions about the policy or would like more advice towards extending your home in The New Forest National Park give us a call on 01794 524 200 or email at

Written by Laurence Wright March 2024
Architect and Director of V&L.
Passionate about residential design and creating dream homes for our clients