Two listings, both a mile apart, both marketed as "one-acre estate parcels." One will support a 6,500 square foot main house with a pool, a detached studio, and a circular motor court. The other will cap out closer to 4,900 square feet before the Planning Commission asks hard questions, and the owner will owe a pathway dedication on top of it. The lot size is identical. The buildable envelope is not.
For buyers moving from Atherton, Woodside, or Palo Alto, this is the single biggest source of surprise in a Los Altos Hills transaction. The town runs on a set of formulas that translate raw acreage into something narrower, and a softer 2026 market gives a prepared buyer real leverage to price that gap into the offer.
The formula that quietly resizes every hillside lot
Los Altos Hills does not zone by minimum square footage the way most Peninsula cities do. It calculates a Lot Unit Factor for the parcel and then applies the Maximum Development Area formula in Article 5 of the municipal code:
| Average slope | Maximum Development Area |
|---|---|
| Under 10% | LUF × 15,000 sf |
| 10–30% | LUF × [15,000 − 375(S−10)] sf |
| Over 30% | LUF × 7,500 sf |
MDA covers everything artificial on the ground: the house footprint, the driveway beyond the first 100 feet, patios, pool decks, tennis courts, sport courts. Maximum Floor Area is a separate cap on gross interior square footage, with a 5,000 sf floor when the LUF exceeds 0.50.
The Town's own MFA/MDA worksheet works the math on a one-acre parcel at 14% slope and lands at 12,339 sf of MDA. That is the entire ground-level artificial surface budget for the property. Move that same acre to a 22% slope and the formula bleeds off roughly a third of it. Nothing about the listing photos will tell a buyer this. The slope average, calculated from a topographic survey, will.
The practical read: on a hillside parcel, the buyer is not really buying acreage. They are buying LUF, and LUF is a function of net area and slope. Ask for the worksheet before the inspection contingency drops. The Planning Department keeps prior worksheets on file at Town Hall for many parcels, and any competent civil engineer can produce a fresh one from a boundary and topo survey.
Why the pathway easement is a line item, not a footnote
Los Altos Hills has been building its Master Path Plan since the town incorporated in 1956, and it funds the extensions through a development impact fee. Under the pathway ordinance, any site development permit for a main residence, an ADU, or a cumulative addition of 900 or more habitable square feet over a rolling ten-year window can trigger one of three obligations: dedicate an easement and build the path, dedicate an easement for a future path, or pay the pathway fee.
The fee sits at $53.00 per linear foot of the average width of the property, calculated by Town engineering staff, and the Pathways FAQ puts the typical parcel fee around $11,000. On September 18, 2025 the City Council passed a resolution updating the Pathway Development Impact Fee and pathway construction requirements, with the ordinance amendments adopted October 16, 2025. Buyers underwriting a project that will trigger the 900 sf threshold should confirm the current schedule with the Planning Department before finalizing an offer.
There is a second-order effect that matters more than the fee. A pathway easement dedication carves a permanent public corridor across the parcel. It cannot be blocked with a fence, wall, or gate, and the town's fencing code specifically excludes those from pathway easements. If privacy is the reason the buyer chose Los Altos Hills in the first place, the location of an existing or planned pathway on the Master Path Plan is a due diligence item, not a curiosity.
Sewer or septic changes the price of the same house
The 2026 Sewer System Management Plan reports 1,912 residential connections to the Town's sanitary sewer as of April 2026, with the remainder of the town's parcels on septic. Roughly half of Los Altos Hills is on septic, and the wastewater that does flow through the collection system is treated at the City of Palo Alto's Regional Water Quality Control Plant.
Whether a specific parcel stays on septic is not a choice. Article 1 of the sewage chapter requires any new residence or major addition to connect to the public sewer if a main is within 200 feet of the property line, with narrow engineering exceptions where gravity flow will not work. For a buyer, that 200-foot line drawn on the utility map is the difference between plumbing straight into a lateral and permitting a full on-site sewage disposal system through Santa Clara County's Department of Environmental Health.
The septic requirements themselves shape both the build and the calendar. Installation and repair are prohibited between November 1 and April 1. Drainfields are not approved on slopes over 50%, and slopes over 20% require additional soil investigation. Two drainfields at 50% of the required size each, interconnected with a diversion valve, plus a reserve area sized for 100% future expansion, all need to fit inside the buildable portion of the lot without encroaching setbacks or easements. On a constrained hillside parcel, the septic reserve area is often the reason a proposed footprint has to shrink.
The calendar constraint most buyers miss
Grading in Los Altos Hills is prohibited between October 1 and April 30 without a special approval, and grading hours during the permitted window are limited to 8:00 a.m. through 5:30 p.m., Monday through Friday. A buyer closing in October on a knock-down candidate is not starting site work in November. The clock effectively runs from May to September of the following year for any earth-moving scope, which compresses the schedule for pool excavation, driveway regrading, septic install, and foundation prep into a single dry-season window. Sequencing this well is the difference between a two-year project and a three-and-a-half-year one.
What the softer 2026 market lets a prepared buyer do
Los Altos Hills is not the frenzied market it was in 2023. Redfin's tracking through May 2026 shows a median sale price near $5.3M over the trailing three months, down 9.46% year over year, with median days on market at 15 versus 9 the year before. Zillow's ZHVI puts the average home value at roughly $5.26M, down 4.4% year over year.
The interpretive point is not the direction of prices. It is the widening spread between list and close, and the extra week of decision time. A buyer who arrives with a completed MDA calculation, a distance-to-sewer measurement, and a Master Path Plan overlay knows exactly which listings are priced as if the buildable envelope matches the lot line, and which are priced on the true envelope. That gap, on a $5M-plus parcel, is negotiable dollars.
A short due-diligence sequence before the offer goes firm
- Pull the parcel's most recent MFA/MDA worksheet from Town Hall records, or commission a fresh one from the boundary and topo survey.
- Overlay the parcel on the Master Path Plan and note any Class 1, 2, or 3 designations touching the property.
- Measure the distance from the parcel line to the nearest sewer main. If it exceeds 200 feet, request the Santa Clara County DEH septic file and confirm reserve area capacity.
- Confirm average slope across the developable portion, not just the parcel average. The formula punishes slope steeply between 10% and 30%.
- Check the list of streets under excavation moratorium if the project will involve utility trenching in the road.
- Sequence any earth-moving scope inside the May-through-September grading window.
Common questions
Does a fully below-grade basement count against MFA? Basements that meet the town's definition are exempted from floor area, but the definition is specific: at least 75% of the basement perimeter must be wholly underground, and the finished floor above cannot exceed 28 inches over adjoining grade.
Do ADUs eat into the MDA budget? Up to 800 square feet, a new attached or detached ADU is exempt from both MDA and MFA. Anything over 800 square feet counts toward both caps.
How tall can the primary residence go? Up to 32 feet with the standard setbacks increased per code, and a hard ceiling of 35 feet overall.
Los Altos Hills rewards preparation. The parcels that reward it most are the ones where the listing sheet and the buildable envelope disagree, and where the seller has not done the math yet. Straser Silicon Valley works these transactions across the Peninsula and can produce the underwriting before the offer is written. Request a white-glove consultation to walk your target parcel through the LUF, path, and septic analysis before your next tour.
