Good Houses
Loft conversion cost calculator
Step 1 of 3
Where's the loft?
A postcode gives you figures for your actual area. You can also skip this and get a regional estimate.
Before you contact anyone, go and stand in your loft with a tape measure. Head height decides whether a conversion is possible at all, and it is the one thing you can check yourself in five minutes. Measure from the joists you are standing on up to the highest point of the roof. Two metres is the minimum, and around 2.4m is comfortable. Bear in mind that insulation and a new floor take some of what you measure today.
Two things then move this figure more than anything else. The first is the type of conversion: a rooflight is the cheapest route, a dormer costs more, and a hipped roof may need converting to a gable before the space works at all. The second is whether you want an ensuite, which brings plumbing and a great deal of drilling with it, so it is rarely a small addition.
How the conversion type sets the price
The rate per square metre depends on the type of conversion, and the gap between them is large. A rooflight conversion works with the roof you already have. A dormer, a hip-to-gable or a mansard each rebuild progressively more of it, and the cost follows that rather than the floor area alone.
That rate is applied to your floor area, adjusted for the finish you choose and for local labour costs, then the extras you have said you need are added: an ensuite, the staircase, and the structural calculations and building regulations the work requires.
The result is always a range, never a single figure. A precise number would be a false promise, because two builders quoting the same job honestly can land a long way apart. Everything is rounded rather than shown to the pound, for the same reason.
Every figure behind this comes from a table we maintain by hand and review on a schedule. The date of the last review is shown on this page, so you can judge how current it is rather than take our word for it.
What the loft figure covers
The figure covers:
- The conversion itself: structure, insulation, floor, roof alterations and finishing to the level you chose
- The staircase up to the new floor
- An ensuite, where you have said you want one
- Structural calculations and building regulations
- A party wall agreement where the work affects a shared wall
What the loft figure leaves out
Not in the figure:
- Making good the floor below where the new staircase lands
- Reworking the landing or losing a bedroom to accommodate the stairs
- Rewiring or upgrading the consumer unit if the existing supply cannot carry another floor
- Moving a water tank or boiler currently living in the loft
- Strengthening below if the existing structure will not take the load
- Redecorating the rest of the house after the disruption
Why loft conversions cost more in some regions
Labour is the part of a loft conversion that moves most by region, so it is the part we adjust. Every area carries a labour multiplier, and the calculator applies it to the labour element rather than to the whole figure, because materials cost broadly the same everywhere, and scaling them by region would overstate the difference.
Where we hold published data for your area, that area's own multiplier is used. Where we do not, the calculator falls back to the figure for your region and the page says so. It will never quietly borrow a nearby city's numbers and present them as local. A London figure on a Cumbrian postcode is worse than no figure at all.
Taking this to loft specialists
Take it to real quotes, and treat it as the shape of the answer rather than the answer. Get three written quotes from specialist loft firms as well as general builders, and compare what each one includes before you compare the totals, because the cheapest number on the page is often the one leaving the most out.
If a quote comes in below our range, that usually means a newer business building a reputation, or work that cuts corners. Above it usually means limited local supply, or a firm with higher overheads. Both can be the right choice. The question worth asking is what is included, not why the number differs.
Worth asking every firm you speak to:
- Which conversion type are you quoting for, and why that one?
- Where exactly will the staircase land, and what does that cost downstairs?
- Is the head height sufficient without lowering the ceilings below?
- Does the price include building regulations sign-off?
- How long will the roof be open, and what protects the house meanwhile?
Common questions
Will my loft convert at all?
The usual limit is head height rather than floor area. If there is not enough height under the ridge, the roof has to change, which is what moves you from a rooflight conversion to a dormer or mansard. A builder or architect can tell you in one visit.
Does a loft conversion pay back?
Loft conversions tend to return more of their cost than most work, because they add a room without taking garden. The calculator estimates the value added at your area's prices and shows it next to the cost, including when the gap goes the wrong way.
What do I actually lose?
Storage, usually. The loft was doing a job. And landing space downstairs for the new staircase. Neither shows up in a cost estimate, which is why we say it here.
Is this a quote?
No. It is an estimate built from typical costs, and no contractor is bound by it. Use it to know whether a quote you have been given is roughly where it should be, and to walk into that conversation knowing what questions to ask.
Why do you show a range instead of one number?
Because one number would be misleading. The answer to "what will this cost" is a range, and where you land inside it depends on your property, your area and who you hire. Any tool giving you a single figure to the pound is telling you something it cannot know.
Do you save what I type in?
No. The calculation runs and the answers are discarded. Nothing is written down unless you explicitly ask us to email or save a result. We do not build a profile from calculator use.
How current are the figures?
The date of the last review is shown on this page. The costs behind the calculator are maintained by hand rather than scraped, and reviewed on a schedule.