Too hot in summer, too cold the other 10 months, badly converted lofts represent extremely poor value for money - sometimes costing twice the original estimate to get right.
"If a loft has been built badly or without proper planning permission, the owner may need to spend the same amount of money as it cost to build," says Morris McGruer, business manager for national loft builders Econoloft.
"I've seen dormer windows installed which could blow down in high winds, and some lofts are simply too dangerous to use." Mr McGruer has also met loft owners who have showers that they can't use. "The builders installed the shower under the sloping roof where the user can't actually stand up."
Some people minimise various risks from the outset. After Mary Anne Perkins and Anthony Susman were married, their home suddenly had to contend with frequent visits from Dr Perkins's adult child and Mr Susman's three teenagers.
They installed a WC in their guest room, using a two-door arrangement which allows en suite access from the room itself or from the hall. Many months later, they decided to convert the loft.
They made initial inquiries in January, and six months later their home has seen no tool noisier or nastier than a tape measure.
They consulted the Yellow Pages and interviewed several firms: "We selected Econoloft because of their price, and partly the way the other companies presented themselves. One changed tack midstream about the construction method, and another was more concerned with slagging off other firms," says Dr Perkins, senior lecturer at Kingston University.
"About 10 days later, two men from Econoloft came to take measurements," explains Mr Susman, a solicitor. But instead of receiving plans, they received another visit from an Econoloft man about five weeks later. "He measured and remeasured the stairs, exchanging information on his mobile phone with Econoloft for about a half-hour."
They then received the full plans, wrote to Econoloft to resolve a few points, and now confront their next hurdle, which involves the neighbours on either side of their mid-terraced house: "We are sending the plans to each of them. We have to get their approval under a new law. It stops people doing work which may damage the neighbour's property," says Mr Susman.
If they knew they were going to convert the loft, would they have installed the loo in the guest room? "A lavatory in our loft would have been very expensive and very hard to get right," says Dr Perkins. Some houses have plumbing which can be extended upwards fairly easily and cheaply. Others don't.
For some lofts, says Mr McGruer, the only feasible WC is a macerator, a contraption which is best described by what it is not: it is not a flush toilet. And generally it is not as good or as nice or as reliable as a flush toilet.
When the builders arrive in earnest, Mr Susman expects his loft to be converted in a fraction of time that the paperwork is taking - a matter of weeks, not months.
This slow, cautious, clever approach to loft conversion is strongly endorsed by Julian Owen, an architect: "I recommend a consulting structural engineer. Any roof structure has to be treated with great respect. The forces involved are comparatively quite large. Roofs have a tendency to spreading, to push out. The big problem is getting enough space through the ceiling ties to put the stairs in."
Plenty of builders will sidestep building regulations and professional consultations, but this can be a false economy, says Mr Owen, a director of ASBA, Associated Self Build Architects. "If you don't involve outside professionals it will be expensive because of low quality. Besides, the planners may learn about it anyway. Roof spread can be extremely serious and may not show up until months or years later. You also run the risk of leaks, of poor insulation leading to condensation, of poor ventilation leading to rot, and even fire danger."
Colin McBride hired a consulting engineer when he converted his loft as part of a larger refurbishment.
Mr McBride purchased the dilapidated downstairs flat in a two-flat Edwardian house in Sydenham, near Crystal Palace, in 1987. "I intended to renovate it and sell it on after about a year, but the upstairs flat became available and I bought that one as well." He now owned the freehold, and he decided to convert the property into five flats.
"I have no architectural training, but it was relatively easy to do a site survey of the existing rooms and design around that. I hired a structural engineer to make sure that my alterations were structurally feasible," says Mr McBride. Among other things, the building's foundations needed to be reinforced so that they could take the extra weight of an additional storey supported by steel.
"The engineer used my footprint sketches for his drawings which we sent to the local council," he says. It was not all smooth sailing. Mr McBride's plans called for a greenhouse at the very top: "I had problems getting permission to put the conservatory on the roof. A few years ago, roof terraces were unusual. Today it's the norm. I fought for six months to get consent, but it was worth it. And they sent the district surveyor around periodically to check on the work."
The glorious roof-terrace apartment is his own. "It has two double bedrooms and an L-shaped living room which opens onto the conservatory, which opens onto a 20 by 10-foot garden. It's my private haven."
Havens are where you find, or make, them. "I wouldn't mind doing up another property, so I will sell this one for the right price," says Mr McBride.
ASBA can be contacted on 01924 873873 and Econoloft on 0800 269765.
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