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Building Regs

Building Regs

Conservation of fuel and power in existing dwellings

All Replacement Windows Shall Achieve:
  • Whole window U-value of at least 1.6 (W/m2K)

  • Window energy rating of band C or better

  • Note: No longer an you generally comply with IGU’s with a centre pane U-value of 1.2 (W/m2K). Special circumstances apply.

All Replacement Doors:
  • Where a glazed panel within a door is equal to or greater than 50% of the entire door area (including the immediate frame but not adjacent glazed panels)

  • Doors within the scope of AD L shall have a U-value not exceeding 1.8 (W/m2K)

  • All other doors require a U-value not exceeding 1.8 (W/m2K)


If the window is re-designed (not exactly the same number and type of openers), ventilation could be made worse, therefore, you must check you meet the following requirements for purge ventilation:
  • For hinged or pivot window that opens 30 degrees or more, or for sliding sash windows, the area of the opening sash should be at least 1/20th of the floor area of the room.

  • For hinged or pivot opens less than 30 degrees, the area of the opening sash should be at least 1/10th of the floor area of the room.

Background Ventilation

Trickle Ventilators:
The guidance has not changed since 2006; the following applies:
  • If trickle ventilators are fitted in the window that is being replaced then a trickle ventilator must be fitted to the new window or an appropriately sized air brick fitted. (Two stand locking handles are not acceptable as an alternative to trickle ventilators).

  • If no trickle ventilators are fitted and two stage locking handles are fitted in the windows being removed then either;

    • Trickle vents can be fitted as an alternative, or

    • Two-stage locking handles can be fitted, or

    • Appropriate air bricks can be fitted.

  • If no ventilation is provided in the windows being removed then a number of options are available to the consumer. Consideration should be given to the fitting of:

    • Trickle vents, or

    • Two stage locking handles, or

    • Air bricks.

    • Matters should not be made worse and therefore the customer can in these circumstances opt not to fit ventilators as set out above.

Replacing Vertical Sliding Sashes

When replacing vertical sliding windows it is important that compliance to Approved Document N (Safety) is achieved and that the replacement windows do not make the situation worse in the cases of Approved Document F (Ventilation), and B (Fire Egress). Unless the existing window exceeds current Building Regulations regarding Approved Document F (Ventilation), and B (Fire Egress), in which case both can be reduced to comply with the existing regulation.


It is essential when considering whether a structure is a conservatory and therefore not part of the main structure of the property to understand the definition as quoted in the Building Regulations.
To be classed as a conservatory, a structure needs to have:
  • Not less than ¾ of the roof area made from translucent material, and

  • Not less than ½ of the external wall area made from translucent material

  • Either unheated, or heated by a system with own and separate heating controls (ref. Approved Document L1 CL 1.59(b)), and

  • Be separated from the main residence by “external” doors

If the structure meets this definition, in full, then it is exempt from the requirement of Approved Document L1 (Fuel Conservation).

Furthermore if the structure is less than 30m2 and at ground level, then it is generally not necessary to obtain building control approval (Ref: Schedule 2, Class vii) It should be noted that whether or not a structure is deemed to be a ‘conservatory’, or an extension to a dwelling, the requirements of Approved Document N (Safety Glazing) apply.

If the structure is deemed to be a conservatory, then all doors and windows between the conservatory and the main residence must, when replaced, comply with Approved Document L1 (Fuel Conservation), and to the documents to which it refers. The conservatory is not to be registered with FENSA but replacing the dividing doors and windows needs to be.

If the structure does not meet the full definition of a conservatory it does not have an exempt status and must comply with Approved Document L1 (Fuel Conservation). In this situation any doors and windows between the residence and the extension will be classed as internal and exempt from Approved Document L1 (Fuel Conservation)
fensa registered

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