Annexe or extension: what is the difference?

Annexe and extension get used for each other, by me as well, and across the kitchen table that does no harm. You mean the same thing either way: more room on the same house, without moving. As soon as a drawing, a quote or a permit check comes into it, it does start to matter. Then they are different things, with different consequences for the structure, for the council and for what you pay. Below I pull them apart, so you know what you are talking about when someone comes to measure up.

Extension: an existing room gets bigger

With an extension you enlarge a space that is already there. The rear facade opens up, and what you build onto it becomes part of the room behind. No threshold, no connecting door, no separate function. Your kitchen or your living room simply gets deeper.

The facade opening up is the feature everything else hangs on. A rear facade is often load-bearing, and whatever that wall was carrying has to be taken over. That calls for a steel beam, and a structural engineer to calculate what that steel has to be able to do. In the budget it is not a detail but one of the heavier items. What else goes into an extension like that, from groundwork to skirting, is on extensions and annexes.

Annexe: a new space against the house

With an annexe you set a space against the house that stays its own. A roof of its own, usually a door of its own, often a function of its own: a utility room, a store, a study or a bedroom on the ground floor. The existing facade can largely stay where it is.

If that facade stays closed, the heaviest structural work falls away. Against that, an annexe always gets an outer skin of its own: a roof that has to be watertight, a facade that has to be finished, and a join with the existing house that has to stay neat once both parts start moving differently.

In practice it is rarely black and white. Plenty of annexes end up with a wide set of double doors through to the living room, and then something does come out of the facade after all, only less of it. That is fine. You just want to know that with those doors you have bought yourself a piece of extension.

Outbuilding: it does not touch the house

An outbuilding stands on its own. A garden room, a shed down the back, a detached garage, a workshop. Its own foundation, its own roof, its own facade, and no join at all with the house.

In building terms that makes it the simplest of the three, because the join with an existing house is exactly the part where the precision work sits. What comes on top is the route to it: water, electrics and sometimes a waste pipe have to cross the garden, and the garden has to be whole again afterwards. Putting that right is garden work and paving, and you are better off planning it in straight away than arranging it later as a separate job.

Watch out that the council does not always take on the word outbuilding. Often there is one collective term covering everything you add to your house, attached or detached, and it all gets added up. So a shed that has stood there for years can count towards what you are still allowed to build now.

Sunroom: glass is the main material

A sunroom is not a fourth category. It is a way of building one. If it is attached to the house, in building terms it is an extension or an annexe, only with glass as its main material. The question that counts after that is not what you call it, but whether it becomes an unheated space for spring and autumn or a heated part of your house. In that second case the same insulation requirements apply as for the rest of your home, and that changes the floor, the glazing and the draught sealing. There is more on that under sunrooms and skylights.

Why it matters to the council

For the assessment your choice of word makes little difference. The council looks at where the structure is going to stand, how deep and how high it will be, how big your plot is, how much of it is already built on and what has been laid down for your street.

It only goes wrong when you assume a category yourself and conclude from that it is permit-free. I think along beforehand about what is likely to need assessing and where I bring in a structural engineer, but the application runs in the owner’s name, so you submit it yourself. I supply the supporting information.

Why it matters for the structure

The hinge point is always the same: does a piece of load-bearing wall come out, yes or no. With an extension the answer is nearly always yes, with an annexe often not, and with an outbuilding never. Hanging off that is whether calculations have to be made, whether steel has to be ordered and carried inside, and whether the existing house has to be propped while the work is on.

The second point is the roof. An extension ties into the existing facade, an annexe gets a roof of its own that connects onto the house, and an outbuilding stands clear. Every join is a place where water can get in, so the more joins, the more detailing. That work belongs with the roofing and not with the finish you end up looking at.

Why it matters for the price

For the same number of square metres an annexe often works out better than an extension, because the opening and the beam fall away. Do not count on that blindly though, because an annexe does get a facade and a roof of its own. The items shift, they do not disappear.

What holds in every case is that the dearest parts sit under and behind what you end up seeing. Groundwork, foundation, structure, roof and glazing weigh heavier than the tiles and the plastering. That is why I would rather not name a figure before I have measured up, and why the quote says item by item where the uncertainties sit.

So which word do you use

The one you already have in your head. Describe what you want to end up with rather than what it is called: a kitchen that is no longer boxed in, a utility room added, a workspace away from the house. From that I work out which form it becomes, and if what you are asking for is technically something other than what you call it, I will just say so. If you do not know yet which way it should go, put your plan to me first before anyone starts drawing.