Owner intent: thick walls form one continuous hatched body (seamless L and T), grow both ways from the room centreline, fills/light stay inside the inner contour, displayed area is the clean floor, and sun wedges start at the two room-side corners of an opening.

Code: src/wall-thickness.ts, render in src/houseplan-card.ts / src/space-render.ts. Sun: src/sun.ts. Tests: test/wall-thickness.test.mjs, demo/smoke_wall_thickness.mjs.

1. Model

Per space: walls: [{ key, cm }]. Key = quantised midpoint + direction (modulo 180°). Config always stores centimetres. Open boundaries refuse thickness. One physical stretch has one thickness (atomic collinear spans when neighbours overlap only partially). Atomic keys are storage only while a real geometric break requires them: once an original edge is entirely solid with one thickness, normalisation writes its single whole-edge key again. Likewise, touching/overlapping open_spans of the same room pair are stored as one span; pair ownership remains a hard boundary so Split can derive exact open_to links.

Degrade unmatched keys silently on write. Resize / undo / scale re-key all touched spans in the same transaction and keep walls in the resize snapshot.

2. Growth (centreline ±½)

Every thick wall grows half outward and half inward from the polygon edge (outer and shared alike). Silhouette is wider than the polygon by cm/2 on outer walls. Paper and the content frame grow under that outer half.

3. Body render

Production body is the ring outset(poly, half) − inset(poly, half) per room, unioned across rooms so shared and T junctions show one continuous body with no internal seams (as in the reference plan). The body is painted in two layers: a solid fill from global settings.fill_colors.wall_fill (default opaque white, with its own opacity) under the diagonal hatch whose stroke matches the wall outline. Normally neither replaces the other. When the body is thinner than 3 CSS px on screen, the shared full/static render policy suppresses only the hatch so it does not collapse into noise; the solid fill remains. Mitre joins; bevel when the mitre spike exceeds MITRE_LIMIT × thickness.

Openings cut the body full-depth; jambs cap the cut; window glass mid-tunnel; door swing from the inner face. Association uses wall direction ≈ opening angle (mod 180°), then nearest span — never a perpendicular neighbour at a T. At an open_span endpoint, real arms owned by different room contours receive the same bounded mitre patch as arms from one contour; a virtual T therefore has one clean outer corner rather than two butt caps forming a step. The virtual segment itself remains centreline-based. View paints it before the wall body, which masks the part inside each adjoining thick jamb; editors paint it after the body so its full stored extent and live preview stay visible.

4. Floor, fills, light, area

Inner contour = inset(poly, half[]).
Room fills and glow are clipped to the inner contour (not painted into the wall hatch).
Glow leaving through a door uses the clear rectangular opening tunnel: its sector is the intersection of the doorway spans at the near and far inner faces. The two jamb returns therefore clip off-axis light; a zero-depth wall keeps the original centreline sector.
Displayed m² = area of the inner contour (clean floor). Wall-length rulers and opening anchors stay on the centreline.
With no thickness, inner = poly (parity with pre-thickness behaviour).

5. Sun

Wedges do not draw through wall bodies. Their full source span is translated from the centreline by half the wall depth along the receiving room's inward normal, so both side rays begin at the two room-side corners of the window opening. Clip to the receiving room's inner contour. d = 0 keeps the previous centreline/full-span wedge. hide_openings hides the symbol only.

6. Tool / hooks / i18n

Plan-editor tool «Wall thickness»; hover whole wall; cm/in from HA; empty/0 clears; apply-to-room. Hooks: data-hp="wall". i18n en/ru.

Draw with thickness. The Plan toolbar's Walls button carries its session thickness field immediately on the right (default 15 cm, or inches when HA is imperial). Closing a new room outline writes that cm onto every new edge that does not already have one; shared stretches that already carry a neighbour's thickness are left alone. Empty / 0 leaves the room thin. Live thick preview follows the rubber-band while drawing. Split does not use this field. The Wall thickness tool remains for later edits.

7. Out of scope

Decor-line thickness, per-side finish, auto-from-backdrop, plan-wide default.

8. Testing

Unit: ring closed at corners; half-out; inner area; atomic partial shared; virtual-T mitre; angle-aware opening; thick-door tunnel clipping; whole and atomic rekey after edge/scale. Browser: seamless frame; fill not in hatch; m² drops with thickness; a partial virtual stretch, its solid thick remainders and Undo move as one real resize; the virtual rubber band paints above the real body; sun starts at the room-side opening corners; nav mode restores after can_write; a 1 cm body uses solid-only in both full and static cards while a 20 cm body keeps its hatch.

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Wall thickness — the spec