The definition is a "dwelling" includes a dwelling-house and a flat.

For clarification, if the main purpose of a building is to be a "dwelling" then the Regulations will apply to all replacement windows in that building. Eg. sheltered accommodation - since its main purpose is to provide dwellings for people, work on communal areas would count as work on dwellings.

All contracts (including Local Authority or Housing Association) for replacement windows and doors in dwellings can be covered by FENSA.

If I use Low-E glass will it affect the colour of inserts such as Georgian Bars in the sealed unit?

There is no reason why inserts inside the sealed unit should suffer more or less discolouration where Low-E glass is used.
However, viewed from the inside, Low-E may give a different coloured effect, when they are viewed through the glass. The high performance, 'neutral low E glasses' have the least effect on colour.

The requirement of the Regulations specifies the overall performance of the window, or the glass. There are no exclusions for decorative effects.

The Building Regulations Approved Documents N (Safety Glazing) and L (Thermal Insulation) are prime requirements for replacement windows and doors. Other elements of the Building Regulations A (Structure), F (Ventilation), B (Means of escape), J (Combustion Appliances and Fuel Storage Systems), M (Disabled Access) must not be compromised by the replacement installation.
Compliance with the Gas Safety (Installation and Use) Regulations 1998 is essential.

If there is any doubt about the original compliance with the Regulations, the installer must collect evidence of the features of the original installation. It may be that ventilation in the 'partially open' position will be acceptable in some circumstances (see the guidance in Approved Document F). However, it will be good practice to offer trickle ventilation to customers to improve ventilation and help with the control of condensation. If there is any doubt about achieving compliance with regard to any of these matters, the installer should contact their local Building Control Department.

Bay windows are included in the FENSA Scheme. This means that FENSA registered companies are able to certify and must ensure that bay window installations meet Building Regulations and do not compromise the load bearing ability of the bay. Therefore all appropriate poles and spreader plates must be in place to ensure conformance.

A bay window is a multi faceted structure including mullions.
Please note that for the purpose of the FENSA Scheme a bay window is registered on the database as one window.

A patio door is made up of one facet and therefore = 1 door

Not by FENSA, the Local Authority Building Control Department is responsible for non-dwelling works.

Windows and door configurations are given in GGF Data Sheet 2.2 January 2002 'Window and Door U values: Provision of Certified Data'. Trickle vents, Georgina/leaded effects and separate sills are not included. The assessment and testing should be carried out on operational windows.

FENSA applies to the replacement windows and doors to the Building Regulations for England and Wales. The Building Standards for Scotland, Northern Ireland and other parts of the British Isles may be different.

As the U value performance of the doors with less than 50% of their area glazed is not a Building Regulation requirement for dwellings there is no need to certify. If in doubt whether Part L2 applies, please your local Building Control Department.

When fitted into an unheated area, which is not part of the house, the windows need not comply with the Building Regulations Approved Document L. However, you may decide to standardise your product range and the information supplied with all windows, in order to reduce variations and specials.