Ancillary services
Inspections beyond the standard: radon, mold, sewer scope, and wood-destroying organisms
A standard home inspection is a visual, non-invasive look at the readily accessible parts of a house. Some of the things buyers worry about most, radon in the basement air, mold behind a wall, a collapsing sewer line, termites in the framing, sit outside that definition on purpose. They are not oversights. The standards that govern a home inspection name most of them as explicit exclusions, because each one needs an instrument, a lab, a camera, or a licensed specialty the general inspection does not carry.
This page is a companion to what a home inspection actually covers, and what it does not. That page walks through the required scope and the general exclusions. This one goes into the specific services buyers order separately, what each one is, why the standard leaves it out, when it is worth ordering, and what it typically costs. The two documents behind every exclusion cited here are the Standard of Practice from the American Society of Home Inspectors and the Standards of Practice from InterNACHI, the two references most state licensing boards adopt or model.
Why "ancillary" is the right word. These are not upgrades to a better inspection. They are separate examinations, often by a separate licensed trade, that answer a question the standard inspection is not built to answer. A general inspector may hold some of these licenses too, but the work is billed and reported on its own terms. Order them alongside the base inspection, not after it, so results land inside your contingency window.
Radon
What it is. Radon is a colorless, odorless radioactive gas that seeps up from soil and rock into the lowest livable level of a house. It is invisible to any visual inspection and can only be found by measuring the air over time with a continuous monitor or a passive test kit sent to a lab.
Why the standard excludes it. Both standards name it directly. The InterNACHI Standards of Practice state that the inspector is not required to determine the presence of airborne hazards including radon (nachi.org), and the ASHI standard excludes environmental hazards in soil, water and air. Radon is an air measurement, not a visible defect, so it falls outside a visual examination by definition.
When to order it. Effectively always, and especially if the house has a basement or a slab-on-grade lowest level. The U.S. Environmental Protection Agency recommends homes be fixed if the radon level is 4 pCi/L or more, and because there is no known safe level, that Americans consider fixing their home for levels between 2 and 4 pCi/L (epa.gov). A short-term test during the inspection period tells you which category the house falls in before you waive your contingency.
Typical cost. A professional radon measurement runs about $100 to $200 when added to a home inspection, and roughly $125 to $400 as a standalone test, according to home-services cost data (homeinspectioncost.net; homeguide.com). If a test comes back high, a mitigation system averages about $1,200 to $2,500 (fixr.com), which is a useful number to hold when negotiating who installs it.
Mold and moisture
What it is. A mold assessment looks for active fungal growth and the moisture source feeding it, usually combining a visual survey with surface swabs or air samples analyzed by a lab. It answers what a stain actually is and how far it has spread, which the eye alone cannot.
Why the standard excludes it. The InterNACHI standard states that the inspector is not required to determine the presence of mold, mildew or fungus (nachi.org), and the ASHI standard excludes molds and mold-like substances along with other potentially hazardous substances. A general inspector will note a stain that looks like mold and recommend follow-up, but identifying it and sampling it is a separate service.
When to order it. When the general inspection flags visible staining, a musty odor, a history of leaks or flooding, or elevated moisture readings, and whenever an occupant has unexplained respiratory symptoms. A clean general inspection with no moisture findings usually does not warrant a mold assessment on its own.
Typical cost. A mold inspection with a visual assessment plus sampling typically costs about $300 to $700, and can reach $1,000 for large homes or widespread growth, per home-services cost data (homeinspectioncost.net; homeguide.com).
Sewer scope (sewer lateral inspection)
What it is. A sewer scope runs a flexible video camera down the home's sewer lateral, the private pipe that carries waste from the house to the municipal main or the septic tank. It reveals cracks, root intrusion, bellies, offsets, and blockages that are entirely underground and entirely invisible from inside the house.
Why the standard excludes it. The ASHI standard lists underground items and underground drainage among things the inspector is not required to inspect, and both standards limit the plumbing inspection to readily accessible, visible piping. A buried lateral is neither visible nor accessible, so it is out of scope. A general plumbing check confirms fixtures drain; it says nothing about the pipe in the yard.
When to order it. On any home more than roughly 20 to 25 years old, any home with mature trees near the sewer line, and any home on an older clay or cast-iron lateral. This is one of the highest-value ancillary tests because a failed lateral is expensive and hidden.
Typical cost. A sewer scope typically costs about $150 to $300 and takes 30 to 60 minutes, per home-services cost data (homeinspectioncost.net). Set that against the repair exposure: sewer line repairs average in the low thousands of dollars and a full replacement can exceed $10,000 depending on length and site conditions (amerisave.com), which is the whole reason the scope pays for itself.
Wood-destroying organisms (termite and pest)
What it is. A wood-destroying organism inspection, often called a WDO or termite inspection, looks specifically for evidence of termites, carpenter ants, powderpost beetles, and wood-decay fungi, and produces a written report on a standardized form. It is frequently required by lenders, and VA and FHA loans often mandate it in areas where the pests are common.
Why the standard excludes it. The InterNACHI standard states the inspector is not required to determine the presence of evidence of rodents, birds, bats, animals, insects, or other pests (nachi.org), and the ASHI standard excludes wood destroying organisms among the life forms it does not cover. Identifying an active infestation and grading the damage is a licensed pest-control function in most states, distinct from a general inspection.
When to order it. Whenever your lender requires it, in any region with active termite pressure, and any time the general inspection notes mud tubes, frass, hollow-sounding wood, or prior treatment. In much of the South and coastal markets it is close to routine.
Typical cost. A WDO or termite inspection typically costs about $75 to $150 in most markets (homeinspectioncost.net), though a real-estate transaction that requires a formal report and clearance letter often runs higher, roughly $100 to $325 (angi.com).
Others worth knowing about
Beyond the big four, several more services sit outside the standard and become relevant depending on the house.
| Service | Why the standard leaves it out | When to order it | Typical cost |
|---|---|---|---|
| Chimney (Level 2) | The general inspection covers only readily accessible portions of the chimney; the interior flue is inspected by camera | Any home with a wood-burning fireplace or stove, or before first use after purchase | About $250 to $600 (fixr.com) |
| Private well and water quality | Named exclusion in the ASHI standard; InterNACHI does not require inspecting wells or springs (nachi.org) | Any home on a private well; lenders on FHA, VA and USDA loans usually require a water test | Well inspection about $300 to $600; lab water test about $100 to $350 (homeguide.com) |
| Septic system | Named exclusion in the ASHI standard; InterNACHI does not require inspecting septic systems (nachi.org) | Any home on septic. In Massachusetts a Title 5 inspection is required at transfer | Massachusetts Title 5 inspection about $400 to $700 on average (title5specialists.com) |
| Swimming pool and spa | Pools and their equipment are outside the required scope of both standards | Any home with an in-ground or above-ground pool or spa | About $200 to $500 (homeguide.com) |
On septic specifically, Massachusetts is a useful example of how state rules can turn an ancillary service into a legal requirement. Under the state's Title 5 regulations, a septic system generally must be inspected at the time a property is transferred, and a system that fails must be repaired or replaced, which is a rule the general home inspection standards never impose (mass.gov). Wherever you buy, check whether your state or your lender converts one of these optional services into a mandatory one.
How to decide what to order
- Start from the house, not the menu. A slab home with no fireplace, city sewer, and municipal water needs a very different set of add-ons than a 1960s house on a well, septic, and a wood stove. Match the services to what the property actually has.
- Read the general inspection's limitations section first. Anything the inspector could not reach, or flagged and referred out, is a direct pointer to which ancillary service to book.
- Book inside your contingency window. Radon in particular takes days to measure, and lab results for mold and water testing take time to return. Ordering late is the most common reason a buyer loses the leverage a finding would have given them.
- Treat each report as its own negotiation input. A high radon result, a cracked lateral, or an active WDO finding each carries a known repair cost, and a known cost is exactly what a price or repair credit is built from.
None of these services makes the base inspection less valuable. They extend it into the places a visual, non-invasive examination was never designed to reach. Knowing which ones a given house needs, and ordering them early, is the difference between a report that describes the house you can see and a full picture of the house you are actually buying.
Sources
- Standard of Practice, American Society of Home Inspectors (ASHI). General limitations and exclusions, including underground items, environmental hazards, and wood destroying organisms. homeinspector.org
- Standards of Practice, International Association of Certified Home Inspectors (InterNACHI). Exclusions for radon and airborne hazards, mold and fungus, pests, wells, and septic systems. nachi.org
- Radon guidance and action level, U.S. Environmental Protection Agency. Recommendation to fix homes at 4 pCi/L or more and to consider fixing between 2 and 4 pCi/L. epa.gov
- Title 5 septic system regulations and transfer requirements, Commonwealth of Massachusetts. mass.gov
- Specialized home inspection cost data (radon, sewer scope, termite/WDO, mold), home-services cost index. homeinspectioncost.net
- Radon, mold, well, and pool inspection cost guides, home-services cost reference. homeguide.com
- Termite inspection cost guide, home-services cost reference. angi.com
- Chimney and radon mitigation cost guides, home-services cost reference. fixr.com
- Sewer scope inspection cost and repair-exposure guide, mortgage-education reference. amerisave.com
- Massachusetts Title 5 septic inspection cost guide, septic-inspection specialist. title5specialists.com
Related
- What a home inspection actually covers, and what it does not, the companion reference on required scope and general exclusions.
- Outline of an inspection, from the ALLSAFE archive: how one Massachusetts practice described its own on-site coverage.
- The full ALLSAFE archive index.