When a house stops fitting the family living in it, the first instinct is to look at the property listings. The second, once stamp duty, legal fees and the price gap to the next size up have sunk in, is to look at the house you already own. For most families the realistic choice comes down to two projects: build out with an extension or build up with an attic conversion.
They solve different problems, cost different amounts, and disrupt your life in different ways. This guide sets them side by side so you can decide before you pay anyone for drawings.
In short: if you need bedrooms, convert the attic. If you need living space, extend. If you need both, do the attic first, because it is cheaper per square metre and you can live in the house while it happens.
The two options side by side
| Rear or side extension | Attic conversion | |
| Best for | Kitchen-diner, family room, utility, downstairs bedroom | Bedroom, ensuite, home office, teenager’s room |
| Typical cost per m² | Higher: foundations, walls, roof and glazing are all new | Lower: the structure and roof already exist |
| Build time | 3 to 6 months | 6 to 10 weeks |
| Disruption | High; kitchen often out of action for weeks | Moderate; mostly contained upstairs until the stairs go in |
| Garden lost | Yes | None |
| Planning | Often exempt below a size limit; larger ones need permission | Usually exempt if no dormer faces the road; dormers often need permission |
| Value added | Strong, especially open-plan kitchen-living space | Strong if it counts as a habitable bedroom with compliant stairs and headroom |
Will your attic actually convert?
Not every attic can. Check these four things before going further.
- Head height. Measure from the top of the ceiling joists to the underside of the ridge. You want at least 2.4 metres before insulation and flooring eat into it. Below about 2.2 metres the project usually means raising the roof, which changes the budget entirely.
- Roof structure. Older cut roofs with open rafters convert easily. Modern trussed roofs, recognisable by the W-shaped webbing, need steel beams to replace the trusses. Possible, but more expensive.
- Stair position. The new stairs have to land somewhere, usually above the existing flight. If the only option takes a chunk out of a bedroom, the net gain shrinks.
- Fire safety. A third storey triggers requirements for fire doors, a protected escape route and interlinked alarms. It is routine work but it must be in the quote.
What drives the cost of an extension
Quotes for apparently similar extensions can differ by half. These are the reasons.
- Ground conditions. Poor ground, nearby trees or drains under the footprint mean deeper or engineered foundations.
- Glazing. A wall of sliding doors and a rooflight can cost as much as the blockwork for the whole extension.
- The kitchen. If the extension houses a new kitchen, the fit-out can equal a third of the build cost on its own.
- Steel. Knocking through the back wall to create open plan needs structural steel and an engineer’s design.
- Access. A terraced house where everything comes through the hall costs more than a detached one with side access.
- Finish level. Underfloor heating, polished concrete and flush thresholds all add up quickly.
| Project | Typical size | Budget guide |
| Basic attic conversion with rooflights | 20 to 30 m² | Lowest of the four |
| Dormer attic conversion with ensuite | 25 to 35 m² | Roughly 1.5 times a basic conversion |
| Single-storey rear extension | 20 to 40 m² | Roughly 2 to 3 times a basic conversion, before the kitchen |
| Two-storey extension | 40 to 70 m² | Highest overall, but the lowest cost per m² of any extension |
Per-square-metre rates vary too much by region and year to quote responsibly in an article. Get three itemised quotes against the same drawings, and be suspicious of any figure given before someone has seen the site.
Choosing the builder
The contractor matters more than the design. A good one will save an average design; a bad one will ruin a good one.
- Ask to see a finished job and a job in progress. The finished one shows quality; the live one shows how they run a site.
- Insist on an itemised quote, with provisional sums clearly marked, so you can compare like with like.
- Check insurance and registration, and who is responsible for certifying compliance with building regulations.
- Agree stage payments tied to milestones, never to dates, and hold a retention until snagging is complete.
- Look for a contractor who does both types of project. A firm that handles extensions, conversions and renovations, as OS Holding does in Dublin, has no reason to steer you towards one or the other and can price them against each other honestly.
Planning and paperwork
Many rear extensions and rooflight-only attic conversions fall within exempted or permitted development, but the limits on size, height and distance from boundaries are specific, and they differ between Ireland and the UK and between local authorities. Dormers facing the road, anything on a protected structure, and anything in a conservation area almost always need permission. The Planning Portal has interactive guides for England and Wales; in Ireland, your local authority planning office will confirm in writing whether a proposal is exempt.
A checklist before you call anyone
- Write down what you are short of: bedrooms, living space, storage, or a place to work. Be specific.
- Measure the attic from joist to ridge, and photograph the roof structure.
- Sketch where a new stair could land without losing a room.
- Measure the garden and decide how much of it you are willing to give up.
- Find your drains. A manhole in the footprint of an extension adds cost and time.
- Set a total budget including fit-out, then hold back ten to fifteen per cent as contingency before you brief anyone.
Frequently asked questions
Which adds more value? An extra bedroom usually moves the valuation more than extra living space, but only if it meets the regulations to be called a bedroom. A non-compliant attic room is valued as storage.
Can we live in the house during the work? During an attic conversion, yes. During an extension, yes until the back wall comes out, after which many families move out or live without a kitchen for several weeks.
Do I need an architect? For a simple attic conversion, an engineer and a competent contractor are often enough. For an extension, a designer earns their fee in layout and light.
What is the most common regret? Under-budgeting the finish. The structure is rarely what goes over; the kitchen, flooring and glazing are.
Decide what kind of space you are short of, check the attic against the four tests above, and get both options priced from the same contractor. The right answer is usually obvious once the two quotes are on the same table.
