Part G Water Calculator — AD G Appendix A Water Efficiency

The Building Regulations 2010 regulation 36(2) states: 'The requirement referred to in paragraph (1) is either (a) 125 litres per person per day; or (b) in a case to which paragraph (3) applies, the optional requirement of 110 litres per person per day, as measured in either case in accordance with a methodology approved by the Secretary of State.' The approved methodology is Appendix A of Approved Document G, the water efficiency calculator, and that is what this page runs. The figure is a calculated potential consumption derived from the fittings specification, not metered usage.

Worked from Table A1 and expressed after the 0.91 normalisation factor, so these are the litres per person per day the headline figure actually moves by. Shower 8 l/min instead of 12: 4 x 4.37 = 17.48, so 15.9 saved. WC 4/2.6 dual flush instead of 6/4: 20.60 becomes 13.54, so 6.4 saved. Basin taps 5 l/min instead of 8: 4.74, so 4.3 saved. Bath 140 litres instead of 170: 3.30, so 3.0 saved. Kitchen sink taps 6 l/min instead of 8: 0.88, so 0.8 saved. The shower is by far the biggest single lever, because its use factor of 4.37, or 5.60 where there is no bath, is the largest the table applies to any flow rate.

Table A1 has a column (4) row for each of: the WC, taps other than kitchen and utility taps, the bath, the shower, kitchen and utility sink taps, the washing machine, the dishwasher, a waste disposal unit, and a water softener. This page works all of those except the water softener. Two further rows sit below the fitting rows and are subtracted from the total: row (6), the contribution from greywater, and row (7), the contribution from rainwater. This page treats both as zero. So exactly three things are left out, and they are not the same kind of thing: the water softener is a FITTING row whose litres per person per day come from Table A3 and need the regeneration figures plus an occupancy count, while greywater and rainwater are DEDUCTIONS taken from Tables A4.6 and A5.5. Appendix A itself excludes four things on purpose: bidets (paragraph A2(b)), taps on baths, because paragraph A2(c)(iv) says that water is taken account of in the use factor for baths, external taps, because paragraph A2(j)(i) assumes a fixed allowance of five litres per person per day, and swimming pools and spa hot tubs (paragraph A4). Where a water softener or any water re-use applies to your dwelling, complete the full Appendix A tables.

Yes. Regulation 36 applies to every new dwelling, and paragraph 2.13 explains that regulation 37 requires 'a notice specifying the calculated potential consumption of wholesome water per person per day relating to the dwelling as constructed' to be given to the building control body. Paragraph 2.14 says that notice must in most cases be given no later than five days after the completion of the building work. The fittings installed have to match the ones used in the calculation, and that requirement is not in a numbered paragraph: it is item (d) of the unnumbered Performance statement facing the G2 guidance, which says Requirement G2 will be met for new dwellings if 'a record of the sanitary appliances and white goods used in the water consumption calculation and installed in the dwelling is provided along with sufficient other information enabling building owners or occupiers to maintain the building and its services so as to maintain the water efficiency of the building', adding that 'in this context, relevant white goods are washing machines and dishwashers'. Paragraph 2.1 is a separate one-sentence point about where the numbers come from: 'The water used by sanitary appliances and relevant white goods in a new dwelling should be calculated using the manufacturer's declared value for water consumption of each of those appliances and white goods.'

Regulation 36(3) says the optional requirement applies where the planning permission for the work specifies it and makes compliance a condition of that permission. It is not something a local authority can apply generally, and it is not something you opt into: paragraph 2.8 confirms it only bites where that planning condition exists, and paragraph 2.9 requires the person carrying out the work to tell the building control body when it does. Select 110 above when your permission carries that condition.

Not for the fittings. Table A1 column (4) is headed 'Litres/person/day = [(1) x (2)] + (3)', so every fitting row is already per person and household size cancels out. A calculator that divides that total by the number of occupants is not running the approved method. Occupancy is not absent from Appendix A, though, and it would be wrong to say it appears only once. It is an input to three tables, all of them ones this page does not model: Table A3 box (d), 'Number of occupants served by the system', for water softeners; Table A5.1 box (e) for rainwater collected under the BS 8515 intermediate approach, where box (f) is 'Daily rainwater per person (litres) = [(d)/(e)]'; and Table A5.2 box (b) for the detailed approach. In each case it is a bedroom count rather than a free choice. Paragraph A22(c): 'Enter the number of occupants into box (e), which can be based on two occupants in the first bedroom and one occupant in each additional bedroom. A studio flat should assume two occupants.' Paragraph A15 says the same for the water softener table.

Both steps are rows of Table A1. Row (8) is a normalisation factor of 0.91 applied to the total calculated use after any greywater and rainwater savings, which is row (9). Row (10) then adds a fixed 5.0 litres per person per day for external water use, giving the row (11) total. Paragraph 2.3 puts it plainly: the requirement includes a fixed factor of water for outdoor use of 5 litres/person/day. Skipping either step gives a number that is not the regulated quantity, so it cannot be submitted to building control.

Yes, but which table you are held to depends on which target you are working to, and there are two of them. Paragraph 2.5 allows a fittings approach as an alternative for the 125 litres requirement, and paragraph 2.10 does the same for the 110 litres optional requirement. For 125, paragraph 2.6: 'Where the fittings approach is used, the water consumption of the fittings provided must not exceed the values in Table 2.1.' Table 2.1 is WC 6/4 litres dual flush or 4.5 litres single flush, shower 10 l/min, bath 185 litres, basin taps 6 l/min, sink taps 8 l/min, dishwasher 1.25 l/place setting, washing machine 8.17 l/kilogram. For 110, paragraph 2.11 says exactly the same about Table 2.2, and Table 2.2 is tighter on five of the seven rows: WC 4/2.6 litres dual flush, shower 8 l/min, bath 170 litres, basin taps 5 l/min, sink taps 6 l/min, dishwasher 1.25 l/place setting, washing machine 8.17 l/kilogram. So a 10 l/min shower and a 185 litre bath satisfy Table 2.1 and fail Table 2.2, and Table 2.2 offers no single-flush WC at all. Exceed any row of whichever table applies to you and the water efficiency calculator must be completed instead. It must also be completed where a shower is not to be provided, or where a waste disposal unit, a water softener or water re-use is to be provided. The regulation 37 notice then states 'Less than 125 litres/person/day using fittings approach' (paragraph 2.7) or 'Less than 110 litres/person/day using fittings approach' (paragraph 2.12).