MEP Plans in Atherton, CA

Atherton project facts we design around

County	San Mateo County
Title 24 climate zone	Climate Zone 3, confirmed by ZIP for every project
Electric utility (most addresses)	Pacific Gas and Electric (PG&E)
Permit review	The City of Atherton building department
ADU permits issued, 2021–2025	184 (#142 of 450 cities)
ADU approval timeline	Approve or deny within 60 days of a complete application (state law)

ADU building activity in Atherton

Atherton issued 184 ADU building permits from 2021 through 2025, the 142nd most among the 450 California cities that reported ADU permits. Permits went from 32 in 2021 to 45 in 2025.

2021	2022	2023	2024	2025
32	25	35	47	45

Source: California Department of Housing and Community Development, Annual Progress Report data (ADU building permits by year).

Title 24 in Climate Zone 3

• Windows: U-factor 0.27 or lower (no SHGC limit in this zone), with total window area up to 20% of floor area.
• Solar PV sized by the CEC formula kW = (floor area x 0.628) / 1000 + 1.12; no PV is required if that is under 1.8 kWdc, which for a single ADU works out to about 1,083 sq ft or smaller.

Climate Zone 3 requirements in full →

HVAC design for Atherton

The California Energy Commission's design-day data for Atherton gives the outdoor conditions a Manual J load calculation uses: a summer design temperature of 82°F and a winter design temperature of 23°F.

Summer design (1%)	82°F dry bulb / 64°F wet bulb
Winter design (median of extremes)	23°F
Daily temperature range	27°F
Elevation	50 ft

Both seasons matter here. The heat pump is usually sized for the 82°F cooling condition and then checked to confirm it still covers the heating load at 23°F.

Electrical service from Pacific Gas and Electric (PG&E)

If the load calculation shows you need a service upgrade, a second meter or a relocated service, Pacific Gas and Electric (PG&E) handles that through its Your Projects portal, separately from the building permit.

ADU electrical service with Pacific Gas and Electric (PG&E) →

Plumbing and sewer

The plumbing plan shows the route, slope and cleanouts of the connection to the existing sewer lateral. Some Bay Area cities also have private sewer lateral inspection rules, so ask whether your project triggers one.

Pricing is flat statewide: ADU plans from $995, full ADU MEP package from $2,295, Title 24 add-on $240. See what's in a complete ADU MEP set.