Christopher Alexander, in his book ‘A Pattern Language’ points out that any balcony which is smaller than 6 foot square is virtually never used. So this includes almost all balconies you see and explains why they are never in use (and therefore a waste of energy, money and resources). Particularly silly is the so-called Juliet balcony. See this Pattern Language example
Balconies need careful consideration to avoid causing a thermal bridge through the floor of the balcony into the internal floor[for the purposes of part E of the Approved Documents] - Any floor that is not a separating floor (see separating floor). of the room it serves. This has caused a great deal of redesign of balconies over the last decade or so.
One option is to make balconies free standing with minimal connection to the building. A steel structure standing on its own foundations is a way of achieving this. With concrete floors there is the possibility of using insulated connectors between the floor and balcony (such as the Ancon Isolan product)
Another way is to jetty a timber floor out through the wall to form the floor of the balcony. This requires careful detailing to ensure that the area of structural timber linking the inside and outside is minimised so that that the thermal bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a cold bridge. see more on thermal bridging is not too great.
Masonite beams may achieve this as they have a relatively small cross sectional area. It also requires attention to airtightness at this point and a way of ensuring that moisture cannot penetrate back into the building from the balcony floor.
The balcony floor must be totally watertight and properly drained to protect the joists.
Another approach is to hang the balcony from the roof, possible by stainless steel cables or struts. This requires extra detailing by a structural engineer (probably entailing extra strengthening at the eaves) and then either an understanding of stainless fasteners etc. or specialist steel fabrication and erection.
The Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place). See more on the regulations cover thermal bridging in the Approved DocumentsApproved documents (England) are detailed publications which come under the English Building Regulations. They are based on tried and tested methods of building and if you follow them you are assured of complying with the Regs. The equivalents for Scotland are the Technical HandbookUnder the Scottish Building Regulations, the Technical Handbook gives construction principles, which, if you follow them guarantee compliance with the Regulations, for Wales: the Approved documents (Wales), and for N.I. the Technical BookletsUnder the Northern Ireland Building Regulations, the Technical Booklets give construction principles, which, if you follow them guarantee compliance with the Regulations L1A for new dwellings.