The short answer
A loft conversion is usually the better choice if you need an extra bedroom or office and your roof space is suitable, because it adds space without using any of your garden. A single storey extension is usually better if you want a bigger kitchen, dining area or family room, or if your loft is too low or awkward to convert. Neither is cheaper in every case, so the real prices homeowners paid, shown separately on this page, are a useful check against any quote. Your house, your layout and what you want the space for will decide it more than any general rule.
How they differ
A loft conversion turns the unused roof space into a habitable room. Depending on the roof, it may involve fitting windows into the existing slope, or adding a dormer or similar structure to create more headroom and floor area. You also need a new staircase, which takes up some space on the floor below, usually from a landing or bedroom.
A single storey extension adds a new room at the back or side of the house, on the ground floor. It needs foundations, new walls, a roof and usually some opening-up of the existing house so the old and new spaces connect.
This leads to different results in daily life:
- A loft conversion normally gives you a quiet room away from the main living areas. It suits bedrooms, offices and play rooms.
- An extension sits alongside your existing living space. It suits kitchens, dining rooms and open-plan family areas, and it usually gives easy access to the garden.
- A loft conversion leaves your garden untouched. An extension uses some of it.
Both types of work usually need to meet building regulations, and either may need planning permission depending on your property and what you are building. Many homes have some allowances that make planning permission unnecessary, but these have conditions, and they do not apply in every case. Flats, listed buildings and homes in conservation areas often face tighter rules. Check with your local council before you commit. If the work affects a shared wall or is close to a neighbour's boundary, the Party Wall Act may also apply.
Cost: what makes each more or less expensive
Neither option has a fixed price advantage. What you pay depends on the details of your house and the specification you choose.
Loft conversion
Costs tend to be lower when the roof already has enough height, the structure is straightforward and you are happy with a simple layout. They rise when the roof needs significant structural change, when a larger dormer is needed for headroom, or when you add an en-suite, which means more plumbing and more finishing. The staircase and fire safety measures also add cost, and the work has to be done to a high standard because you are building above the rest of the house.
Single storey extension
Costs tend to rise with the size of the extension, the ground conditions, the type of foundations needed and how much of the existing back wall has to be taken out. Large glazed doors or roof lights, underfloor heating and a fitted kitchen can push the total up quickly. Drainage that needs to be moved can also add to the bill. A simple, modest box is generally cheaper than something with an unusual shape or a lot of glass.
In general, per square metre of usable space, an extension often costs more than a loft conversion, because you are building a whole new structure from the ground up. But a small, simple extension can cost less overall than a large or complex loft conversion. Compare quotes for the same specification, and look at the real prices paid on this page rather than relying on rules of thumb.
Lifespan and upkeep
Both are permanent additions and, if built properly, should last as long as the rest of the house. The differences are mostly in what needs looking after.
A loft conversion has a roof and, often, dormer cladding or flat roof sections that need occasional checks. Flat roof coverings on dormers wear out over time and may need replacing before the rest of the structure. Insulation matters a great deal in a loft room. Done badly, it can leave the room too hot in summer and cold in winter.
An extension also has a roof to maintain, particularly if it has a flat roof or large roof lights. Seals on glazing and the condition of gutters and drainage need checking. The foundations and walls should need little attention if they were built correctly, but poor drainage or nearby trees can cause problems over the years.
In both cases, the quality of the build and the materials matters more than the type of project. Keep all your paperwork, including building regulations sign-off and any guarantees, because you will be asked for them when you sell.
When to choose Loft conversion
A loft conversion is the better option when:
- You need an extra bedroom, office or quiet space rather than more living space.
- You want to keep your garden as it is.
- Your roof space already has reasonable headroom and a sound structure.
- Your house layout allows a staircase to be added without losing something important on the floor below.
- Your plot is small, or there are boundary or access issues that make building at ground level difficult.
It is less suitable if the roof is low, the roof structure is complicated, or the only place for a staircase would take over a room you need.
When to choose Single storey extension
A single storey extension is the better option when:
- You want a larger kitchen, dining area or family room, especially one that opens onto the garden.
- Your loft is too low, or the roof structure makes conversion impractical or very expensive.
- You want step-free access to the new space, for example for elderly relatives or young children.
- You have enough garden to spare and are happy to give some of it up.
- You may want to add a second storey or convert the loft later, and want to plan for it.
It is less suitable if your garden is already small, if the ground is difficult, or if you mainly need another bedroom.
Questions to ask a tradesperson
- Is my loft suitable for conversion, and what would need to change in the roof structure?
- Where would the staircase go, and what would it take from the floor below?
- Will the work need planning permission, or can it go ahead under permitted development? Who handles the application?
- Who deals with building regulations, and will I receive the completion certificate?
- Is the quote fixed, and what is and is not included, such as electrics, plastering, decorating and flooring?
- Do I need a structural engineer, and is their fee included?
- Will the Party Wall Act apply, and who arranges the notices?
- How long will the work take, and how much will I be able to use the house in the meantime?
- What guarantees or insurance cover the work once it is finished?
- Can I speak to, or see, similar projects you have completed recently?