Almost every domestic extension in Britain is one of a small number of shapes, and the shape is chosen by the plot far more often than by the brief. Knowing which shapes your house allows, before an architect is appointed or a builder visits, saves a lot of money on designs that were never going to be built.
The eight shapes of house extensions UK homes get, and what each is for
Rear, side return, wrap around, two storey, over structure, front, garage conversion and loft conversion. Rear buys depth and darkens the middle. A side return buys width where a narrow plan needs it most. A wrap buys both and pays for the corner. Two storey buys a bedroom for roughly the same roof and foundation. Building over an existing single storey wing buys a room with no new groundworks. Front is the hardest to consent. A garage or loft conversion buys space with no new footprint at all, which is usually the cheapest space in the house.
Extension alternatives most people skip
Before extending, price the reorganisation. Moving a bathroom, removing one internal wall, replacing a run of units, reworking a staircase or converting a garage often produces the room the household actually wanted, at a fraction of the cost and with no planning question. A garden room gives a workspace or a gym without touching the house, though it is not habitable accommodation unless it is built and insulated as such. And moving house, once stamp duty and fees are honestly counted, is a real option rather than a rhetorical one.
Add on rooms: single room additions
A bathroom addition is usually about drainage before anything else, because a new soil connection and a gradient decide where it can go. A living room addition is usually about light and about whether the new space leaves the old one landlocked. Both are conventional builds that engage the same regulations as anything larger: structure, ventilation, drainage, insulation and, where a new room changes escape routes, fire safety.
Choosing on constraint rather than on taste
Work through what the plot allows first. Boundary distances, the position of the drains and any public sewer, whether the house is a flat or a house, whether permitted development rights have been removed, and how much garden you are willing to lose. Most households arrive with a preferred shape and discover one or two are impossible. Ruling out early is cheaper than designing and withdrawing.
What a lean to extension cost turns on, and why the roof decides it
A lean to is the cheapest shape of single storey extension because its roof is the cheapest roof: one slope, tied into the existing wall at the top, no valleys, no hips and no ridge to form. That is also its constraint, because the pitch is set by the height available under the first floor windows and the depth you want, and a shallow pitch narrows what covering will work. Push the depth out and either the pitch drops below what a tiled covering can take, which sends the roof to single ply, EPDM or standing seam, or the top of the slope rises into the windows above. The quote follows that geometry more than the floor area, which is why two lean to quotes for the same room can differ by the price of a different covering.
Questions people ask about types of house extensions
Which extension type adds the most value?
Usually the one that fixes what the house is missing, most often a second bathroom or a usable kitchen and dining space, rather than the largest one.
Is a garage conversion really cheaper?
Normally yes, because the footprint, the foundations and the roof already exist. The costs that surprise people are the floor build up, insulating a thin wall and replacing the door with a compliant wall and window.
Can I extend a flat?
Householder permitted development rights do not apply to flats or maisonettes, so any extension needs a planning application as well as the lease consents.
Does a garden room count as extra living space?
Only for practical purposes. It is normally an outbuilding, so it is not habitable accommodation unless it is designed, insulated and approved as such.