We're still verifying Montgomery County, MD's answers against the county code.

Where can I put a detached accessory dwelling unit in Montgomery County, MD?

Detached Accessory Dwelling Unit

Allowed by right in 10 districts

What this does not cover

• 281,978 properties are loaded, county-wide. They come from Montgomery County's own parcel boundary layer, one polygon per tax account.
• A different authority counts this county differently, and we do not reconcile the two by dividing one into the other. Our county-wide totals elsewhere are counted from the State of Maryland's SDAT parcel points, which lists 348,669 properties (304,119 of them zoned by the county's own ordinance). That is a different unit of counting — one is a parcel boundary, the other is an assessment point — so the difference between the two numbers is not a measure of what is missing here. We matched the two lists account by account: 277,808 properties are on both. 70,861 are on the state's list only — mostly condominium units and accounts with no separate boundary in the county layer we hold — and 4,173 are on the county's only.
• A further 55,537 properties carry a parcel zone code this page cannot match to a single column of the use table: (none), CRF (4 variants, e.g. CRF-1.25), CRNF (4 variants, e.g. CRNF-0.75), CRTF (3 variants, e.g. CRTF-1.5), EOFF, ILF, IMF, PCC, PD (11 variants, e.g. PD-11), PNZ, PRC, R-H, RT (5 variants, e.g. RT-10.0), RURAL and 1 more.
• 7,361 of those carry a code that looks like one of the districts above and is not one. CRF-1.25, CRF-1.5, CRF-1.75, CRF-2.5, CRNF-0.75, CRNF-1.0 and 15 more — these are separate zone classifications in their own right, not the same zone with something added, and the use table has no column for them. We do not fold them in.
• 36,399 of those carry no zone at all in the county's own zoning layer. That is land the county's own zoning layer does not cover; 82% of it sits in ZIP codes centred on Rockville, Gaithersburg and Poolesville, which are incorporated municipalities running their own zoning rather than the county's.
• Owner filters. We have an owner of record for 280,419 of Montgomery County’s 281,978 parcels, of which 46,476 are owned by a company, trust or other organization, including 4,965 owned by public bodies — governments, authorities, schools, churches. That split is Zonescore’s reading of the owner’s name, not something the county published, and we keep the two apart rather than deciding for you which counts as an investor. 9,695 have an owner whose mailing address is outside MD — read off the end of the county’s own address string, where the state actually is, because “MD” also occurs inside “RICHMOND”. We could read a state on 279,777 of them; the rest are not counted either way. 38,574 are owned by somebody this county writes to at a different address — not the property itself. That is where the tax bill goes, which is not the same as where the owner sleeps — an owner can collect mail at a property they rent out, and one who is written to elsewhere may still live here. We can tell you the two addresses differ; we cannot tell you the owner does not live at the property. 31,695 of them are written to an address inside MD, so the out-of-state filter above cannot see them at all. That count is out of the 257,429 properties where we could compare the two addresses. On 22,990 more we could not — the owner is written to a post-office box, or the county’s own record of the property names a street and no building number — and those are left out rather than counted as living at home. Matching two addresses is Zonescore’s reading, not something the county published: we treat “123 Main St” and “123 Main Street” as one place, ignore an apartment number, and compare the ZIP code rather than the town name — a jurisdiction’s record of a property and its record of that property’s owner often name the same place by different towns, and treating those as two addresses would count somebody living in their own house as an absentee. Owned for (based on the last recorded transfer): under 2 years 27,000+; 2-5 years 39,000+; 5-10 years 58,000+; 10-20 years 62,000+; 20-30 years 49,000+; 30+ years 22,000+. 21,000+ properties have no transfer date we can read and are in no bucket. 65,423 are worth more as land than as building — the county’s own land assessment is at least 50% of its total. 1,549 properties carry no pair of assessments and are left out of that count rather than treated as zero, which would make every one of them look like bare land.

  Owner data from Montgomery County Planning Department's property records, retrieved September 3, 2026.

• 12,300 of the properties above reached their district by the ordinance's own rule for reading a map code, not by the code matching the column outright. Chapter 59, Sec. 1.4.1.D, E and F -- the Zoning Map shows each property's zone classification followed by its maximum FAR and height allowances, and the use table is keyed on the classification. So a property mapped CR-0.5 is in the CR district carrying that FAR and height allowance, and the use table answers it in the CR column. We store the county's code exactly as published and derive the classification when we answer — nothing is rewritten. 63 map codes reach a column this way.
• 9,655 of the properties above are listed as “no ZIP in the county’s record” in the ZIP breakdowns. 1,560 carry no street address at all in the county’s data and 8,095 carry a street but no city or ZIP — so we cannot place them in a ZIP, and we do not guess one. They are still counted in the district totals.
• 451 of them (0.2%) carry a ZIP that does not start 208 or 209 — the prefixes covering 99% of the addresses this county publishes. These are the county’s own values, shown as published; some are neighbouring ZIPs along the county line. We have not audited them, and we neither correct them nor drop them, so the count you see is the count that exists.
• Overlay districts are not modeled here. An overlay can change what a property allows.
• Some cells in this ordinance state two outcomes without saying what separates them; those are shown with their code and their source, and not guessed at.

Built from scripts/build_reverse_index.py, generated 2026-10-11T20:59:04+00:00.