Specifying a stone basin comes down to three numbers and one document set: a fabrication tolerance (the Natural Stone Institute allows ±1⁄16" on face dimensions up to 6'-0", per its 2022 Dimension Stone Design Manual), a material class (ASTM C503/C503M-15 caps marble absorption at 0.20% by weight and sets minimum flexural strength at 1,000 psi), and a lead time you can hold a contractor to. Get those on paper with a shop drawing and a batched sample, and stone becomes one of the easier line items on a bathroom spec — not one of the riskiest.
We carve basins to order every week for designers and architects, and almost every problem we see traces back to a spec sheet that said “marble vessel sink, approx. 20 inches” and nothing else. Here is what the documentation should actually contain.
The tolerances to write into your spec
Natural stone is not millwork. The Natural Stone Institute is direct about why published tolerances exist at all: “Tolerances are established to protect both the buyer and seller of a product or service” (Tolerances in the Dimension Stone Industry, Natural Stone Institute, 2022). If you do not name a standard, that document says it “by default, may be cited as the appropriate industry standard.”
The figures that matter for a carved basin and its surrounding countertop:
| Condition | NSI 2022 tolerance | Metric |
|---|---|---|
| Face dimension, pieces up to 6'-0" long | ±1⁄16" | ±1.5 mm |
| Slab thickness, >1¼" to 2" | ±1⁄8" | ±3 mm |
| Deviation from flat plane, cut-to-size with smooth finish | ±1⁄16" in 4'-0" | ±1.5 mm in 1.2 m |
| Countertop seam width, stone to stone | ±25%, not less than ±1⁄64" | ±0.4 mm |
| Lippage at countertop front edge | 0 | 0 |
| Countertop variation from flat and level | 1⁄8" in 10'-0" | 3 mm in 3 m |
Two of those are worth flagging to your GC in advance. Zero allowable lippage at the countertop front edge means the substrate has to be right — NSI applies the same 1⁄8" in 10'-0" flatness limit to the cabinetry or blocking underneath. And edge perpendicularity is allowed to drift ±1.0° on an exposed smooth edge, roughly 1⁄64" per inch of thickness, which is why a 2" apron can read very slightly out of plumb and still be fully compliant.
For a carved basin specifically, add the dimensions the tolerance table does not cover: bowl depth, wall thickness, drain bore diameter and its centre-to-centre position, overflow (or explicitly, no overflow), and faucet deck drilling. Those are project-specific values and, under NSI clause 6.4, any tolerance you and the fabricator agree in writing supersedes the published one.
Material standards: name the ASTM class
“Marble” on a spec sheet is a commercial term covering stones with very different behaviour. The technical reference is ASTM C503/C503M-15, Standard Specification for Marble Dimension Stone, which sets a maximum water absorption of 0.20% by weight and a minimum flexural strength of 1,000 psi (6.9 MPa) for both Classification I (calcite) and Classification II (dolomite) marble.
That 0.20% ceiling is the number to quote when a client asks whether a marble basin will stain. It is low — but it is not zero, and it says nothing about acid sensitivity. Calcium carbonate etches on contact with acidic cleaners regardless of how little water the stone absorbs. Travertine, governed separately by ASTM C1527, is more porous by nature and its voids may be filled or left open, which is a specification decision, not a defect. If your client wants an unfilled travertine basin, say so on the drawing.
Samples: one is not enough
Ask for a control sample, not a chip. On a heavily figured stone — Calacatta Viola, Rosso Levanto, Arabescato — a 4" square tells you the colour family and nothing about the vein. What you want:
- A photo of the actual block or slab your piece will be cut from, with the cut lines indicated.
- A range sample, ideally two pieces showing the extremes of acceptable variation, so approval covers a band rather than one lucky face.
- Batch confirmation when multiple pieces need to match — a double vanity cut from two different blocks will not read as a pair.
- Finish confirmation: honed and polished behave differently. On dark stone, polished surfaces make etch marks far more visible; honed hides them better but shows water spotting more readily.
Related reading: our custom marble sink guide, the wall-mount basin guide, and the stone sink overview cover the material choices behind these decisions in more detail.
Lead times you can put on a programme
Timing is where stone gets specified out of projects unnecessarily. The 2025 U.S. Houzz Bathroom Trends Study found that 84% of renovating homeowners hire a professional, with general contractors engaged by 45% of them — meaning the basin lead time usually has to slot into someone else's trade schedule, not float freely. The same study put the national median bathroom spend at $13,000, rising to $25,000 for bathrooms of 100 sq ft or more, so the basin is rarely the item worth delaying a programme for.
Our own schedule, for reference on a bespoke piece:
| Stage | Duration | Deliverable |
|---|---|---|
| Quote | Within 24 hours | Fixed price, DDP shipping included |
| Technical drawing | Within 48 hours of payment | Dimensioned drawing for your approval — carving starts only after sign-off |
| Carving | 2–3 weeks | Single block, hand-carved in our Antalya atelier; photo and video before dispatch |
| Delivery | About 1 week | Crated with foam, insured express courier, tracked, duties paid |
Roughly four to five weeks door to door. Build in a buffer for sample approval at the front end, because that is the stage that slips.
Two pieces designers specify most
The spec package, in one list
- Stone name, finish (honed / polished), and for travertine: filled or unfilled
- ASTM class reference — C503 for marble, C1527 for travertine
- Governing tolerance document (NSI Dimension Stone Design Manual, current edition) plus any project-specific tolerance that supersedes it
- Dimensioned drawing: overall W × D × H, bowl depth, wall thickness, drain bore and position, overflow yes/no, faucet drilling
- Weight and support detail — wall-hung pieces need blocking or brackets coordinated with framing before board goes up
- Approved range sample and batch reference
- Delivery terms: DDP or DAP, crating, insurance, site access
If you want us to produce that package for a live project, start at custom order — send dimensions and a stone preference and you will have a fixed quote within 24 hours, and a technical drawing to mark up before anything is cut. Designers working on multiple units can apply to our trade programme for batched stone and CAD files.
Frequently asked questions
What tolerance should I specify for a stone basin?
Unless your project demands tighter, cite the Natural Stone Institute's published fabrication tolerances: ±1⁄16" (1.5 mm) on face dimensions for pieces up to 6'-0", ±1⁄8" (3 mm) on slab thickness between 1¼" and 2". Any tighter tolerance must be agreed in writing with the fabricator, because under NSI clause 6.4 an agreed tolerance supersedes the published one.
How long does a bespoke stone basin take?
About four to five weeks in our workflow: quote within 24 hours, technical drawing within 48 hours of payment, 2–3 weeks of carving, and roughly a week of insured express shipping. The variable is sample and drawing approval on your side, so start the specification early in the programme.
Is marble too porous for a bathroom basin?
ASTM C503/C503M-15 limits marble absorption to 0.20% by weight, which is low. The practical issue is not absorption but acid sensitivity — calcium carbonate etches on contact with acidic products, which is a surface effect rather than a stain. Specify a honed finish and pH-neutral cleaning if the client is sensitive to visible etching.
Can I get CAD files and spec sheets for a specification set?
Yes. We issue a dimensioned technical drawing for every bespoke piece before carving begins, and trade accounts receive CAD files and spec sheets as part of the package. Request them through the trade programme page.
