Building a basement
Emily Brooks explains what you need to know before you create or convert a basement
Going
underground is not always as cheap as other ways of extending, but done well,
it represents good value for money and a healthy return on investment. In an
existing house on a small plot it can be the only answer to getting some extra
space. Meanwhile, self-builders are finally realising that basements are a
cost-effective way of optimising a plot without expending too much extra cost
or effort.
From the
ground up
“There’s
definitely a lot more interest now in having a basement on a new build,” says
Darren Wilks from extension specialists Retreat Rooms. “It’s a way of building
that’s thermally efficient and waterproof – while you’re digging out the
foundations anyway, use the opportunity to dig a bit further and put in that
extra space.” Darren cites gyms, media rooms and home offices as popular uses
for this bonus square footage, but don’t rule out using them for storage alone,
or a laundry or plant room – your possessions will be much easier to access in
the basement than if they were in the loft.
Whether you
decide to use your basement as a principal living space such as a kitchen or
living room depends largely on how much natural lighting is available:
architects can work wonders here, with French doors opening onto sunken patio
areas, for example, or light-well grills (flat slabs of reinforced glass that
can be walked on at ground height). Sloping sites can be a real boon here too,
because a ‘semi basement’ can be built into the slope at the back, opening up
to full-size windows or doors at the front (see case study, opposite).
It’s common
for new-build basements to be made from double-skin masonry filled with
concrete, or reinforced concrete poured on site; these structures are then
waterproofed. Andrew Parkes of Midland Tanking gives a rough estimate of
£600/m2 for this sort of system, which includes excavation, building and
tanking (prices per m2 will drop for bigger builds).
Prefabricated
building methods are also coming into their own: some systems (such as
Glatthaar and ThermoneX) use factory-engineered waterproof panels in pre-cast
concrete. Insulated concrete formwork (ICF) – hollow polystyrene blocks pumped
full of concrete with a special waterproof additive – is an alternative method,
and is cheaper than pre-cast. In both cases, construction is quick and there’s no need for additional insulation.
Excavate
to accumulate
In crowded
urban areas, digging down is often the only option for those who want more
space but don’t want (or can’t afford) to move. Many homes have existing
cellars, and floors can be lowered if there’s insufficient head height
(2m/6.5ft is the regulatory minimum, but 2.5m/8ft will look and feel much more
satisfactory). It’s also possible to completely retrofit a basement – that is,
to excavate one from scratch – and this is naturally the most expensive option of them all, around £2,500/m2
without waterproofing. New-ish on the scene, particularly in London where land
costs more, is the trend for extending under the surrounding garden or
driveway, as well as under the house; the new space can be lit with a reinforced glass roof or light wells.
The world
of basement conversions has been turned on its head in recent years with the
invention of the cavity membrane system. Gone are the days of dank and dark
rooms – now, you’re guaranteed a space that is warm, dry and breathable. The
membrane is fixed behind the walls and floors (think giant bubble-wrap, placed
bubble-side-down); any moisture that comes through is trapped behind, trickles
down into a sump, and is excavated with a pump, which kicks in when the water
reaches a certain level. The membrane is plasterboarded or directly plastered
on top, and you then decorate as usual (see Nottingham case study overleaf).
Installing
cavity membranes is a specialist process, says Steve Hodgson, deputy director
of the Property Care Association (PCA), the trade body for structural
waterproofing companies. “Getting the spec right from the beginning is very important – assessing the structure, the ground conditions and a host
of other factors. And if difficulties do occur during the build, you’ve got to
be able to combat them – with waterproofing you can’t bodge. The minute you cut
corners, you increase the risk of failure.”
An
alternative waterproofing method is the application of several coats of a
waterproof-yet-breathable render to walls and floors. It’s a specialist job
that has to be executed perfectly, and unlike a cavity drainage system, there’s
a drying-out period. However, it’s maintenance free: whereas the pump on a
cavity system will usually need to be inspected once a year,
as Steve says, with a render system, “once it’s up, it’s up”.
Planning
and regulations
A
specialist basement company will take care of the planning, design and build,
but if you’re going it alone, you’ll need to call the various trades separately
– an architect to draw up plans, someone to do the
excavating/underpinning/piling, a specialist waterproofing company, Building
Control etc. The websites of the Basement Information Centre and the Property
Care Association are both good places to start looking for suitable trades near
you.
Planning
laws concerning basements are, according to the government, “evolving and under
review” at the moment – probably because of their lack of popularity until
recent years, they’ve managed to fly under the radar. They’re not formally
included in permitted development rules, so if you’re planning to change your
house’s external appearance (eg by installing a light well) or you’re creating
additional habitable space (ie not just storage space in an existing cellar),
your local planners will want to know.
Regardless
of planning laws, your basement will need to meet relevant Building Regulations
(which are numerous), and if you live in a terrace or semi-detached house, a
party-wall agreement between you and your neighbours is also a must.
Contacts
Retreat
Rooms
0845 652 5997
www.retreatrooms.co.uk
The Basement
Information Centre
01276 33155 www.basements.org.uk
The
Property Care Association
0844 375 4301 www.property-care.org
For more
contacts and the full article read the February 2010 issue of Build It
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