You will need planning permission if it is facing the street. If it is facing your garden, it will be most likely a permitted development expect it is in a conservation area. Check with your local authority.

If its a flat then its likely that your permitted development rights have been removed and therefore a juliette balcony would require planning permission as its an external change.

You should be covered by Permitted Development, but I would advise you to check with your Council Planning Services Help Desk just to be sure. You will also have to inform your local Building Control (Separate Dept.) who will require a drawing and application for checking and site inspections which attract their fees.

The only balconies planners will approve are Juliet type.
If only Building Regulation approval required you could use a flat roof area as a balcony.
With this type of work I would submit a Building Notice.