As an Architect I regularly get asked to design attic conversions but for all the requests only one or two get built any year, why? From a homeowners perspective it must seem the easiest way to increase usable space; after all, the attic has already been built, we just need to put in a stairs and maybe a dormer, right?
Wrong.
An attic conversion poses significant challenges in terms of structure, fire safety, acoustics and insulation. So much so, that I now advise anyone who asks, that a basic attic conversion will need a minimum budget of £30,000 + VAT.
Needless to say, this comes as quite a shock and I'm sure someone will get in touch to tell me how their attic was converted for far less by some local cowboy builder. Wait until they try to sell their house and the surveyors tell them it doesn't comply with the Building Standards!
So what are the main factors that make an attic conversion so complex and expensive, why isn't it simple?
Space, You Win Some and You Loose Some:
All habitable rooms must have permanent access, so if your attic is converted into a bedroom, it needs a permanent stairs. The problems start with the space that stairs takes up on the existing top floor of the house. The stairs must have its own enclosure, because the building standards forbid a fire escape route to go "from a room to another room". In simple terms, if the smoke detector goes off in the middle of the night, the person escaping from the new attic room must be able to leave the attic and not encounter another doorway until they get to the main entrance of the house. This stairs and it's enclosure are large and they usually take up half an existing bedroom on the existing top floor. And if you loose half a bedroom, you have lost all of a bedroom.
Realising this, an attic conversion only makes sense if the conversion provides at least two new bedrooms and not every attic is big enough to accommodate two rooms.
Head Room:
The diagram above illustrates the point. Just because your attic is big enough to stand up in, does not make it suitable for a conversion. The attic in House A is 2.3 meters tall, at its highest point, but still is not suitable.
I am 6 foot 4 (1.9m), much taller than average, and I notice when a space is too low. In my experience most people will stand in the middle of their attic, under the highest point of the roof, and declare there is ample height to accommodate a bedroom. While the building regulation don't specify a minimum ceiling height required for a habitable room, they give plenty of other minimum ceiling heights;
1.8 in front of a toilet, bath or shower.
1.9m in any doorway and fire escape zone.
2m above all points on a stars and landing.
In a attic, under a pitched roof, headroom starts to run out the closer one gets to the eaves. This sloping ceiling, know as a coomb, will very quickly impede the headroom required to comply with the building standards.
In my experience, to make an attic conversion work properly, the stair landing should be as close to the middle of the plan as possible. If the required headroom cannot be provided under the existing roof, then a dormer is necessary.
Dormer Design:
There are two main types of dormer commonly used; pitched roof dormers and flatroof dormers.
While most people prefer the look of a pitched room dormer, it has a serious limitation; It's width is determined by its height, the wider the dormer, the taller it's roof. Under current Edinburgh and Fife planning guidelines a dormer should not be taller than the existing roof ridge, this sets an upper limit on how wide the dormer can be. What happens if it can't be as wide as it needs to be, to accommodate a stairs or a bathroom for example?
The answer is a flat roof dormer, it can be as wide as necessary without affecting its roof height.
There are many other planning guidelines on how to design a new dormer, stipulating distances from the existing ridge, eaves and gable. All of these guidelines will determine the maximum size available. Always bear in mind that even the maximum allowable dormer may not provide enough space in the attic. More than one dormer may be required.
Upgrading:
The existing fabric of an attic will almost certainly require three significant upgrades; structural, thermal and acoustic. These upgrades will have an impact on cost and on usable space, particularly headroom.
Structural upgrades are almost always necessary, particularly when building a dormer or if converting the attic in a modern timber frame house. Newer homes are almost always designed with roof trusses instead of rafters and joists. Trusses are prefabricated from slender pieces of timber, connected together in a large triangular frame. That frame is strong but if any of it's components are cut out, it will fail (the polite word for collapse).
In short, I have yet to work on an attic conversion that has not required steel beams and significant amounts of new timber rafters and joists to strengthen the existing roof and attic floor. Of course making the floor and roof thicker will reduce the overall headroom but it is usually unavoidable.
Thermal insulation will be required for obvious reasons, however the type of insulation is important. The insulation must be installed in the coomb, between the rafters. Cheap mineral wool, the kind your local hardware store sells, won't work unless we install over 300mm (1 foot) of it. Given that rafters are seldom more than 200mm thick, accommodating this insulation would require a huge increase in the overall roof thickness. With limited headroom always an issue, it is best to install synthetic, expanded polyurethane insulation, which achieves a greater degree of insulation while being much thinner than mineral wool. The only issue is cost, it is usually five to ten times the price of mineral wool.
Acoustic insulation has until recently been a consideration only between separate properties. Until a recent change in Section 5 of the Scottish Building Standards, acoustic separation only applied between separate properties. Now however a specific acoustic rating must be provided between rooms in the same property. So the floor of the attic must be upgraded to prevent noise transmission. This requires the timbers in the floor to be at least 200mm thick and packed with dense insulation, the ceiling on the existing top floor to have an additional layer of plasterboard and the new attic floor to have two layers of ply or chipboard, separated by an acoustic quilt. Aside from cost, this increases the floor thickness, further reducing headroom. It also means the entire ceiling of the existing top floor must be re-decorated after the new layer of plasterboard is fitted.
Conclusion:
Attic conversions aren't impossible and sometimes they are the only available option to increase space but they are tricky. Talk to an Architect before you commit to converting your attic. Construction costs change over time, see this page for up-to-date costs for attic conversions.