The key here is section 3.3 in the approved documents which covers several applications and scenarios. The section is detailed in note B below. So, what does this mean as several sections of the approved document are referenced?

Where ANY energy efficiency measures are being applied to an existing dwelling then there is a simplified model detailed in the document for assessing the impact on ventilation provision in an existing dwelling. This simplified process, alternatively PAS 2035 can be used to demonstrate compliance.

However, the sections 3.14 to 3.16 apply when replacing windows

Existing windows with background ventilators

3.14     If the existing windows have background ventilators, the replacement windows should include background ventilators. The new background ventilators should comply with both of the following conditions.

a. Not be smaller than the background ventilators in the original window.
b. Be controllable either automatically or by the occupant.

If the size of the background ventilators in the existing window is not known, the ventilator sizes in paragraph 3.15 may be applied.

Existing windows without background ventilators

3.15      Replacing the windows is likely to increase the airtightness of the dwelling. If ventilation is not provided via a mechanical ventilation with heat recovery system, then increasing the airtightness of the building may reduce beneficial ventilation in the building. In these circumstances, it is necessary to ensure that the ventilation provision in the dwelling is no worse than it was before the work was carried out. This may be demonstrated in any of the following ways

a. Incorporating background ventilators in the replacement windows equivalent to the following.
i.  Habitable rooms – minimum 8000mm² equivalent area.
ii. Kitchen – minimum 8000mm² equivalent area.
iii. Bathroom (with or without a toilet) – minimum 4000mm² equivalent area.

b. If the dwelling will have continuous mechanical extract ventilation, installing background ventilators in any replacement windows which are not in wet rooms, with a minimum equivalent area of 4000mm2 in each habitable room.

c. other ventilation provisions, if it can be demonstrated to a building control body that they comply with the requirements of paragraph 3.2.

Note: If it is not technically feasible to adopt the minimum equivalent areas set out in paragraph 3.15 the background ventilators should have equivalent areas as close to the minimum value as is feasible.

3.16     When windows are replaced as part of the work connected with a material change of use, Section 1 of this approved document should be followed in addition to paragraphs 3.14 and 3.15. 

Existing dwellings – Addition of a room, Conservatory, Wet room or refurbishment of a kitchen or bathroom
(see Note C – for sections 1.42 to 1.44)

For the addition of a habitable room, paragraphs 3.17 to 3.20.

3.17   The requirement for adequate ventilation can be met if background ventilators are used as follows.
a. If the additional room is connected to an existing habitable room which now has no windows opening to the outside, paragraphs 1.42 to 1.44 should be followed
b. If the additional room is connected to an existing habitable room that still has windows opening to the outside, the following conditions apply.
i. If the existing habitable room has a total background ventilator equivalent area of less than 5000mm², paragraphs 1.42 to 1.44 should be followed.
ii. If the existing habitable room has a total background ventilator equivalent area of at least 5000mm², both of the following should be provided.

• Background ventilators of at least 12,000mm² equivalent area between the two rooms.
• Background ventilators of at least 12,000mm² equivalent area between the additional room and the outside.

For the addition of a conservatory, paragraphs 3.21 to 3.24.

3.21 The guidance in this section applies to conservatories with a floor area that exceeds 30m2. Conservatories with a floor area that does not exceed 30m2 are exempt from the Part F requirements.

3.22 The general ventilation rate for a new conservatory and, if necessary, adjoining rooms could be achieved using background ventilators. The guidance in paragraphs 1.42 to 1.44 should be followed regardless of the ventilation provisions in the existing room adjacent to the conservatory.

For the addition of a wet room, paragraphs 3.25 to 3.29.

3.25 When a wet room is added to an existing dwelling, whole dwelling ventilation should be extended and extract ventilation should be provided by one of the following means.
a. Intermittent extract, as specified in Table 1.1, and a background ventilator of at least 5000mm2 equivalent area.
b. Continuous extract, as specified in Table 1.2.
c. Single-room heat recovery ventilator, as detailed in paragraph 3.26.

For refurbishing a kitchen or bathroom, paragraphs 3.30 to 3.32.

3.31 If there is no ventilation system in the original room, it is not necessary to provide one in the refurbished room. However, additional ventilation may be necessary if refurbishment work is likely to make the building less compliant with the ventilation requirements of the Building Regulations than it was before the work was carried out. The guidance in paragraphs 3.6 to 3.13 should be followed for refurbishment that includes energy efficiency measures and paragraphs 3.14 to 3.16 should be followed for window replacements.

Note B – 3.3 For common types of work, the requirements of paragraphs 3.1 and 3.2 may be demonstrated by following the guidance detailed below.

a. For installing energy efficiency measures excluding window replacement, paragraphs 3.6 to 3.13.
b. For installing energy efficiency measures including window replacement, paragraphs 3.6 to 3.13 and 3.14 to 3.16.
c. For replacing windows only, paragraphs 3.14 to 3.16.
d. For the addition of a habitable room, paragraphs 3.17 to 3.20.
e. For the addition of a conservatory, paragraphs 3.21 to 3.24.
f. For the addition of a wet room, paragraphs 3.25 to 3.29.
g. For refurbishing a kitchen or bathroom, paragraphs 3.30 to 3.32.
h. For work done to improve the ventilation of the dwelling that was not triggered by the building work in (a) to (g), Section 1 should be followed.

Note C – (see Note C – for sections 1.42 to 1.44)

1.42   If a habitable room does not contain windows that can be opened (e.g. an internal room), the requirement to provide adequate ventilation can be met using paragraphs 1.43 and 1.44. This guidance can be used if the room is ventilated through either of the following.
a. Another habitable room.
b. A conservatory.

1.43   The habitable room or conservatory should have openings to the outside to provide both of the following.
a. Purge ventilation with a minimum total area given in paragraph 1.29, based on the combined floor area of the habitable rooms (or the habitable room and the conservatory).
b. Background ventilation of at least 10,000mm2 equivalent area.

1.44 Between the two rooms there should be a permanent opening with a minimum area of 1/20 of the combined floor area of the two rooms (see Diagram 1.3).