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Single Storey Extension Building Regulations: The Numbers to Hit, the 25% Glazing Rule and How Approval Works

Single storey extension building regulations apply to almost every rear or side extension in England, even when you do not need planning permission. Planning decides whether you can build it; building regulations decide how it must be built: the foundations, the steel over the new opening, the insulation, the glass, the fire protection near the boundary, the drains and the electrics. This page sets out which parts of the regulations bite on a typical single storey extension, the numbers your design has to hit, how the approval routes work, and where planning and party wall rules overlap.

Which regulations apply to a single storey extension

The Building Regulations 2010 are split into Parts, each explained in an Approved Document published by the government on GOV.UK. These are the ones a building control inspector will check on a typical kitchen or living room extension:

Part O, which deals with overheating, applies to new homes, not to extensions to existing ones.

Part L: insulation and the 25% glazing rule

Approved Document L, Volume 1 sets limiting U-values for new elements in an existing dwelling. In plain terms, the lower the number, the less heat escapes:

Then the glazing limit. The total area of windows, glazed doors and rooflights in the extension should not exceed 25% of its floor area, plus the area of any windows or doors the extension blocks or removes from the existing house. A 5m by 4m extension (20 square metres) that covers an existing 4 square metre patio door could carry up to 5 + 4 = 9 square metres of glazing on the simple route.

That rule is where most glass-heavy designs run into trouble. A wall of bifolds plus a big rooflight can easily exceed it. You can still build it, but your designer then has to show by calculation that the extended home performs no worse overall than it would with a compliant extension, usually by beating the U-values elsewhere or upgrading the existing house. Settle this at design stage; it is expensive to discover on site. Our bifold doors building regulations and rooflight regulations pages go into the detail.

Part B: building close to the boundary

Most London extensions run along a party wall or a garden fence, and the closer a wall is to the boundary, the more fire resistance it needs. A wall within 1 metre of the boundary generally has to be fire-resisting with very limited openings, which is why side windows in a side return are often small or omitted. Where the extension makes an existing room an inner room (one you can only leave through another room), the escape route and alarms need thought too. Our open-plan kitchen fire regulations page covers the common case of knocking through into a big kitchen diner.

Part H: drains and building over a sewer

Back gardens in terraced streets often have a shared sewer running behind the houses. If you plan to build over, or within 3 metres of, a public sewer, you need the water company's consent, usually through a build-over agreement, as well as building control approval. Find out early: a CCTV drain survey costs far less than redesigning foundations, and some sewers cannot be built over at all.

How to get approval

• Full plans application: drawings and specification are checked before work starts and you receive an approval notice. The safer route for anything with steel, a large opening or a tricky boundary.
• Building notice: you give notice at least two days before starting and the inspector checks as work goes. Faster, but you carry the risk of being asked to change work already built.
• Council or registered building control approver: either can supervise domestic work in England. Fees are set by each provider.

Inspections usually happen at the foundation excavation, the damp-proof course, drains, the steel and the finished job. At the end you receive a completion certificate; keep it, because a buyer's solicitor will ask for it. If work was done without approval, see our page on regularisation certificates.

Since 1 October 2023, the dutyholder rules in the Building Regulations have applied to domestic work too. For a homeowner this mostly means appointing designers and builders who are competent for the job; when more than one contractor is involved, a principal designer and principal contractor should be appointed, and if you do not, the duties can fall on you.

Planning and party walls: the other two approvals

Building regulations approval is not planning permission. On a house (not a flat), a single storey rear extension is often permitted development if it is no more than 3m deep for an attached house or 4m for a detached house, no more than 4m high, with eaves no higher than 3m if within 2m of the boundary, and, with other extensions, covers no more than half the land around the original house. The larger home extension scheme allows up to 6m (attached) or 8m (detached) after a prior approval application. Conservation areas, listed buildings and homes with permitted development rights removed are different; our conservation area page explains.

If you dig foundations within 3 metres of a neighbour's building and deeper than their foundations, or build on the line of the boundary, the Party Wall etc. Act 1996 requires notice to the neighbour. See our party wall guide.

Designing the inside at the same time

Extensions are where the regulations and the interior design meet: the steel position sets the ceiling line, the glazing cap shapes the light, and the boundary wall decides where a window can go. Plan the kitchen layout, lighting and heating before the drawings go to building control, not after. For the full picture of what regulations apply when renovating, see our building regulations for renovations hub, or talk to us through the Vertigo Interiors homepage.

Sources: GOV.UK, Approved Document L, Volume 1: Dwellings; GOV.UK, Approved Documents; Planning Portal, extensions; GOV.UK, dutyholders and competence.