The short answer
Often yes, but not as a casual upgrade. A balcony is a new use of the roof, so do not assume permitted development covers it: put the idea to the council's planners before any work. The structure must be strong enough to carry people, the edge needs guarding to Building Regulations, and the waterproofing and drainage have to be redesigned to take foot traffic. A written plan and approvals come before any decking goes down.
What changes when a flat roof becomes a place to sit, from planning questions to structure, guarding, drainage and surfacing.
Planning comes first
A flat roof is built to keep water out. A balcony is built for people to stand and sit on, with a view over the garden and your neighbours. That change of use is why planning matters. Overlooking, loss of privacy and the look of the building are the usual concerns, and they are judged by the council, not the roofer.
Do not assume permitted development covers a balcony. The roof alteration rights in the General Permitted Development Order, Class C, deal with changes to a roof of a dwellinghouse, and the order sets limits on how far a change can protrude. Whether a particular balcony falls inside them is for the council to say, and a flat or maisonette sits outside the house rules altogether. Terraced and semi-detached houses are the most sensitive because the neighbour is so close.
Conservation areas and listed buildings add more rules, and some leases and covenants forbid a balcony outright. Send a sketch to the planning officers at your council before you pay for drawings, and ask whether a lawful development certificate or a full application is the right route. Our page on flat roof extension planning permission covers a related question.
Is the structure strong enough
A flat roof designed only to keep the weather out is built for a light load, with little allowance for people. A balcony has to carry the weight of the surface, furniture, plants and a group of people at once. Joists that were fine for a garage roof may be too small, too widely spaced or too weakened by age to carry that.
Even a heavier new covering prompts the Planning Portal to say an engineer should look first, and a balcony adds far more load than a new covering does. Expect an engineer to look at the span, the size of the joists, the bearing in the walls and how the new decking and rails connect to the structure. If the roof sits over a habitable room, they will also want to check the ceiling below can cope.
It is often cheaper to put the structure right while the roof is open. Our page on a sagging flat roof explains how joists are assessed and strengthened. Replacing structural timbers is building work, so ask your building control body whether the job needs approval, and in Crawley that is Crawley Building Control.
Guarding, access and edges
Once people stand on a roof, the edge becomes a fall hazard, and Building Regulations set rules for protection. A balcony needs a barrier at its edge that is high enough and strong enough to hold a person leaning on it, with gaps small enough that a child cannot slip through or climb. The exact heights and loads are set in the Approved Documents, and your building control body can confirm them for your case.
Access matters as well. Most balconies are reached through a door from a bedroom or landing, which means the threshold, the step down and the upstand under the door all have to work together. A door cut into a wall at roof level needs a lintel or support above it, and a threshold that stays clear of standing water.
Neighbours' boundaries need thought. A screen on the side, where one is wanted for privacy, adds wind load on the fixings and may itself need planning consent. Fixings through the waterproofing are the weak points on any roof, so the barrier is best fixed to the structure with proper flashed posts, rather than screwed through the surface. Do not copy a rail from a garden centre catalogue without checking the design.
Waterproofing, drainage and surfaces
A roof that takes foot traffic needs a covering that can cope with it. Some membranes tolerate walking; GRP is a common choice because it forms a hard, jointless surface, as in our answer on can you walk on a GRP roof. Decking, tiles or pedestals are then laid over the top, so the waterproofing is protected and water can drain below the surface.
Drainage must be thought through. Raised decking hides the outlets and gutters, so there has to be a way to reach and clear them. The roof still needs a proper fall, with water running away from the door. Doors need an upstand tall enough to stop splashing over the threshold, and the balcony floor and decking should sit clear of it.
Insulation counts too, so ask your building control body how Part L applies when the membrane is renewed. A balcony quote from us is a single sum, set after the survey, once the planning and structural questions have answers. For the options on the roof itself, see our flat roofing page, and for a planning picture Building Regulations for flat roofs.
More on this in Flat Roofing in Crawley
Two more balcony questions
Can I just put decking on my flat roof and sit out there?
Laying decking alone will not turn a roof into a safe place to stand. The structure may not be built for people, there may be no guarding at the edge, and heavy furniture can damage the covering. To sit out there regularly, treat it as a balcony: ask your council about planning, have the structure checked, and fit a proper barrier.
Does a flat roof over a garage make a better balcony than one over a room?
Often it is simpler, because there is no heated room beneath and less to go wrong with insulation and moisture. The structure still needs checking, though, and overlooking is a question for planning in both cases. A garage roof may have thinner timbers, so do not take its strength for granted.