Buying a lot in the Denver metro area and planning a custom home on it means you’re buying the ground your foundation will sit on — and that ground can vary dramatically across the Front Range. At RKM Homes we see common regional issues that directly affect buildability and cost: expansive clays that swell and shrink with moisture, shallow bedrock that drives excavation costs, perched or seasonal high groundwater, alluvial deposits in drainageways, and legacy oil-and-gas features or buried utilities on older parcels. Identifying these conditions before you close prevents expensive redesigns, unforeseen foundation work, and drainage or utility conflicts once construction begins.
The essential checks to order during due diligence start with a geotechnical investigation performed by a licensed geotechnical engineer: borings or test pits at the proposed building footprint, laboratory classification (grain size, Atterberg limits/plasticity index), swell/expansion tests, and a moisture-density (Proctor) test to establish compaction criteria. Also request groundwater monitoring to determine seasonal high-water table, permeability/infiltration or percolation testing if on-site septic or stormwater infiltration is anticipated, corrosivity tests (pH, sulfate, resistivity) that drive material choices for foundations and buried pipes, and soil-gas screening for radon or methane where oil/gas activity is suspected. If the lot is sloped or near cut/fill areas, have the geotechnical engineer perform slope stability or shear-strength testing; if contamination risk exists, start with a Phase I environmental assessment and follow with targeted Phase II sampling if warranted. These specific tests give a builder the data needed to size bearing capacity, choose slab, pier, or deep foundation systems, and design grading/drainage solutions.
Practically, schedule the geotechnical scope to cover the planned footprint, driveway, utility runs and any future accessory structures so the report is comprehensive. Typical preliminary reports with a few borings are a straightforward part of due diligence and should be written into your purchase contingency; full laboratory-backed recommendations will follow if the geotech identifies issues. Costs and remedies vary widely, but even limited testing pays off by allowing us at RKM Homes to match foundation type and site work to the real subsurface conditions, produce reliable cost estimates, and avoid last-minute change orders that push timelines and budgets.
What geotechnical tests determine a lot’s foundation-bearing capacity?
As a custom home builder in Denver, the first soil test I insist on is a geotechnical investigation that includes exploratory borings and lab testing. Typical field work is two to four borings for a single-family home site (more if the lot is large, sloped, or has visible fill), taken to a depth that reaches the planned foundation level plus an additional 10–20 feet or until a competent layer is encountered. The lab side should include moisture content, grain-size analysis, Atterberg limits, and either Standard Penetration Test (SPT) or CPT data. From those inputs the geotechnical engineer will give an allowable bearing pressure and predicted settlement numbers—those two outputs directly drive the type and size of footings or the need for piers or a mat foundation.
When you order the report, ask for clear, usable deliverables: a recommended allowable bearing pressure in psf, estimated total and differential settlement in inches, recommended footing widths and depths (or pier sizing/depth), and any site-specific excavation or compaction requirements. Also request recommendations for subgrade preparation for slab-on-grade and for any temporary excavation or shoring if the lot requires it. I use those specifics to price foundations accurately, decide whether to budget for deep foundations or undercut/fill, and to include any mitigation items in the purchase offer (for example, an allowance if undercut and engineered fill will be required).
Plan timing and budget practically: a basic geotechnical investigation with borings and a written report typically takes a day or two on site and one to two weeks for the final report, and costs for a typical Denver single-family lot commonly run in the low thousands ($1,500–$5,000 depending on boring count and lab work). Don’t wait until after closing—have the test done or make purchase contingent on an acceptable geotechnical report. Finally, involve your builder when defining boring locations so they align with your planned foundations; that small coordination step prevents surprises and keeps pricing and schedule reliable.
How do I identify and test for expansive, collapsible, or compressible soils on a potential building lot?
As an experienced custom home builder in Denver, I tell every buyer that these soil behaviors are the most common cause of foundation distress we see: expansive clays heave and crack slabs during wet seasons, collapsible silts settle dramatically when wetted, and compressible organic or loose fills produce long‑term settlement. Before you buy, require a geotechnical investigation targeted at these behaviors. That means borings placed through the planned footprint and around foundation corners, field probing (SPT or CPT) to profile density and stratigraphy, and laboratory tests that include Atterberg limits, grain size, moisture content, oedometer consolidation/collapse testing, and swell tests so the engineer can quantify the risk and predict movement under your proposed loads.
When you order the report (or ask your builder to arrange it), be specific about what you need: a minimum number of borings across the building pad and along slopes or likely fill locations, depth to at least the expected foundation bearing elevation (typically 10–15 feet or down to competent material), and lab testing that addresses plasticity and swell potential. Ask the geotechnical engineer for clear, usable outputs — expected total and differential settlement values, predicted percent swell if wetted, allowable bearing pressure recommendations, and explicit foundation options tied to the observed soil layers. Don’t accept a generic statement; make them quantify the problem because those numbers drive foundation design and cost.
If the report shows problematic soils, there are practical mitigation paths you can budget for and discuss with your builder: undercutting and replacing unsuitable material with engineered fill, lime/cement stabilization for expansive clays, or selecting deep foundations (piers or caissons) or grade beams to transfer loads below active layers. For areas with collapse potential, pre‑wetting and recompaction or structural footings are common remedies. Also plan for long‑term moisture control — continuous vapor barriers under slabs, proper grading, gutters, and irrigation plans — because construction technique and water management often determine whether the soil problems show up after move‑in. Ask your builder to include a geotechnical clause in the purchase contract so you can negotiate remedies or walk away if the mitigation scope exceeds your budget.
How will groundwater levels, the seasonal high‑water table, and site drainage affect building my custom home?
As a custom home builder I always recommend having a geotechnical engineer test for groundwater and the seasonal high‑water table (SHWT) before you buy. That means drilling borings to the planned foundation depth (and a few feet below), installing observation wells or piezometers, and—if possible—taking measurements through or shortly after the spring snowmelt or major rain season so you capture seasonal highs. Ask the report to state the depth to groundwater at the time of drilling and the engineer’s opinion about expected seasonal fluctuation; if the initial borings are inconclusive, insist on repeated observations or temporary monitoring wells so you know what to expect across seasons.
Know that shallow groundwater or poor surface drainage changes foundation choices and costs. If the SHWT is within a couple of feet of the slab or bottom of a basement, you’ll likely need full waterproofing, perimeter drains, a sump system, drainage blankets or engineered backfill, or to raise the finished floor above the SHWT; in severe cases deep piers or pile foundations are required to reach load‑bearing material. For surface drainage, plan final grading to slope away from the foundation — a minimum of about 5% (roughly 6 inches over the first 10 feet) — and include swales or retention features where runoff concentrates so hydrostatic pressure and standing water don’t build up against your foundation.
Practical next steps when you’re under contract: require a current geotech report that documents seasonal groundwater levels and recommends foundation elevations and drainage solutions; if the report lacks seasonal data, add a contingency for installing temporary monitoring wells to measure spring melt. Ask your builder for a line‑item estimate of mitigation measures the geotech may recommend so you can budget appropriately, and insist on clear finished‑floor elevation requirements in the construction documents so grading and foundation work are designed around the SHWT findings.
Do I need a percolation (perc) test to install a septic system on this lot?
From my experience as a custom home builder, you absolutely want a perc or permeability test done before you buy raw land that will rely on an on‑site septic system. A perc test tells you how quickly the native soil will absorb effluent and directly determines whether the property can get a septic permit, how large the drain field must be, and whether you’ll need an engineered alternative (mound system, aerobic treatment, sand filter, etc.). Local health departments and permitting agencies set the pass/fail criteria, so a site that looks perfect from the road can still fail a perc and force costly design changes.
Practically speaking, a perc involves digging test holes or pits at representative locations, soaking them, and measuring soak rates or running laboratory permeability tests. I recommend testing multiple locations across a proposed house and drain‑field area (often 2–4 spots) and doing the work in the season that reflects expected high moisture conditions if possible. Hire a licensed septic designer, soil scientist, or local contractor familiar with county requirements; expect a basic perc to cost a few hundred to around a thousand dollars, while a more involved soil permeability report or engineered septic design will cost more and take longer.
If the perc shows unsuitable soil you have clear, actionable choices: locate the house to reach better soils, budget for an engineered system (which can add tens of thousands to the project), or confirm availability and cost to tie into a municipal sewer if nearby. On any lot I’m considering, I make a perc test a purchase contingency item — insist the seller allow access for testing and have the results reviewed with your builder and the local health department before you close. That step avoids surprises and lets you price realistic septic solutions into your build estimate.
How do I test for soil contamination and environmental issues before buying land for a custom home?
Start with a Phase I Environmental Site Assessment (ESA) to review historical uses, municipal records, and obvious red flags like former gas stations, dry cleaners, or industrial operations; if the Phase I flags potential concerns you’ll need a Phase II ESA with targeted soil, soil‑gas and/or groundwater sampling and laboratory analysis. Ask the environmental consultant up front which contaminants they will screen for (common analytes: petroleum hydrocarbons, VOCs, SVOCs/PAHs, lead and other heavy metals, pesticides, PCBs, asbestos in imported fill, and site‑specific compounds like benzene or TPH) and how many boreholes/soil samples they plan to take and at what depths. Expect a Phase I typically in the low thousands of dollars and a Phase II to range from a few thousand to much more depending on sampling density and lab tests — get a written scope, sample count and chain‑of‑custody procedures before you sign anything.
If laboratory results exceed applicable screening levels you’ll need a clear remediation pathway before building: options I commonly see are excavation and removal of contaminated soil, capping with clean fill and an engineered barrier, in‑place treatment, or installation of vapor‑mitigation systems for volatile contaminants. These outcomes affect schedule, budget, permitting and lender/insurer requirements, so include an environmental contingency in your purchase contract (require seller remediation, an escrow holdback, or a credit). Ask the consultant to provide risk‑based comparison values for residential construction, a cost estimate for remediation scenarios, and any recommended institutional controls or long‑term monitoring obligations that would transfer with the property.
On Denver‑area lots I always check for local red flags: former fuel stations, agricultural pesticide use, proximity to known oil & gas wells or old industrial parcels, and any history of imported fill — these dramatically change testing needs and cost. Coordinate environmental sampling with your geotechnical investigation to avoid repeated drilling and save money, and have your builder review the consultant’s scope so tests match construction concerns (e.g., soil vapor for basements or attached garages). Before closing, obtain the full reports and a remediation/cost plan you’re comfortable with, or walk away — remediating a contaminated parcel after construction is far more expensive and disruptive than addressing issues during the land‑purchase phase.