Two separate regimes. Worth understanding before you start.
Planning permission and Building Regulations are different requirements, assessed separately, with different thresholds and different class numbering.
A project can be exempt from one and not the other. Most confusion in this market comes from treating them as a single question — and from quoting a class number without saying which regime it belongs to.
This page sets out how we describe our products, in plain language. It is general guidance, not advice on your site.
Planning
Governs whether you may build the structure at all, and covers size, siting, boundaries, private open space and use. Exempted development classes are numbered here — for example Planning Class 1, Class 3 and Class 3A.
Building Regulations
Governs how the structure is built and how it performs — structure, fire, energy, ventilation, access. It has its own, entirely separate class numbering, including Building Regulations Third Schedule Class 2.
Product, route and typical size.
| AAVA product | Planning route | Typical size |
|---|---|---|
| AAVA Garden | Planning Class 3 — Garden Structure | ≤25 m² core range |
| AAVA Living | Planning Class 3A — Detached Auxiliary Dwelling | 32–45 m² |
| AAVA Extension | Planning Class 1 — Extension | Up to 45 m² |
Why our core range stops at 25 m².
Planning currently permits garden structures to a larger maximum than the Building Regulations exemption. AAVA therefore focuses its core Garden range at or below 25 m² to simplify delivery.
In practical terms there are two separate thresholds in play. The planning ceiling for a garden structure under Planning Class 3 sits at 30 m². The Building Regulations exemption sits at 25 m².
A garden room between those two figures may still fall within the planning class while no longer sitting inside the Building Regulations exemption. Designing our core range at or below 25 m² keeps both questions aligned, which makes the project simpler and more predictable to deliver.
What "auxiliary" actually means.
A Planning Class 3A detached auxiliary dwelling is used in conjunction with the principal house on the site. It is not an independent dwelling.
It cannot be treated as a separate property. It is tied to the main house, and its use is bound by the conditions attached to the class. Eligibility depends on the site.
Because the space is habitable, it is designed to meet the applicable Building Regulations requirements in full — the Building Regulations exemption that applies to small non-habitable garden structures is not relevant here.
Why 45 m² is a ceiling, not an entitlement.
Planning Class 1 covers extensions, with a maximum of 45 m² subject to cumulative and site-specific conditions.
Cumulative is the operative word. Any extension already built is counted towards the total, so the area available to a new project is often less than the headline figure — sometimes considerably less.
The character of the existing house, the amount of private open space that remains, and the position of boundaries all shape what is achievable.
What we look at on every site.
Exempted development classes come with conditions. These are the ones that most often decide a project.
Article 9 restrictions
A set of restrictions that can remove exemption entirely, regardless of size.
Existing extensions
Previous work counts cumulatively and reduces what remains available.
Boundaries
Proximity to boundaries, and the relationship to neighbouring properties.
Private open space
How much usable open space remains behind the house after building.
Site access
Whether the structure and the plant can physically reach the location.
Intended use
Use restrictions apply. How the space will be used affects the route.
Detached status
Garden structures must remain detached and comply with use restrictions.
Conservation restrictions
Protected structures and designated areas change the position, where applicable.
The house itself
Its form, its history and what has already been done to it.
Use matters.
The regulatory treatment of a garden room depends on how it is used. If your intended use includes business or habitation, we'll assess the appropriate route before proceeding.
We do not tell customers that a garden room can automatically be used as a home office, a business premises or as habitation. Where a project depends on that question, we will say so, and we will establish the position properly before anything is confirmed.
Find out where your site actually stands.
Our site qualification looks at the conditions above against your specific property, before you commit to a design.