Interior design basics

In short

Interior design is the planning, detailing and furnishing of the inside of a building so that enclosed space works for the people who use it - dimensionally, visually, environmentally and legally. It deals with layout and circulation, anthropometry, furniture, finishes, colour, lighting, acoustics, interior services and cost, within shells that are usually already built. In B.Arch it is examined in Design studio juries and in theory papers on building materials, services and estimating; questions on lux levels, the lumen method, reverberation time, furniture dimensions and finish specification recur in university papers and in GATE-AR (Building Services, Construction and Management sections). It is directly employable knowledge: Indian turnkey-interior firms, modular-furniture brands, retail and hospitality chains, real-estate developers' fit-out teams and PMC consultancies hire fresh graduates as junior interior designers, site coordinators and BOQ/estimation assistants.

Explained

Scope, elements and the design process

• Definition: interior design shapes the *enclosed volume* - floor, wall, ceiling planes, openings, furniture, fittings, finishes, colour, light, acoustics and air - as one system. Interior *decoration* is only the surface and movable layer of this.
• Elements of interior space: base plane (floor), overhead plane (ceiling, including false ceiling), vertical planes (walls, partitions, glazing), openings (doors, windows, pass-throughs) and the objects in the volume (furniture, equipment, soft furnishing, art, plants).
• Visual vocabulary applied to interiors: line, shape, form, texture, pattern, colour, light and space; organised by balance (symmetric or asymmetric), rhythm, emphasis (a focal point per room), harmony, contrast, and scale and proportion judged against the human body rather than the street.
• Design process (studio and practice sequence):
  1. Brief and user study - occupants, activities, numbers, timings, storage, budget, statutory limits.
  2. Site measurement - measured drawings of the shell; locate columns, beams, shear walls, ducts, plumbing shafts, slab level, floor-to-soffit height. Nothing structural is cut without a structural engineer's clearance.
  3. Programming - area statement per activity, adjacency matrix, bubble diagram, circulation study.
  4. Concept and zoning - public/private, wet/dry, quiet/noisy; service cores fixed first because plumbing moves least.
  5. Layout development - furniture layout, flooring pattern, reflected ceiling plan (RCP), electrical and lighting layout, elevations, sections, joinery details at 1:10 or 1:5.
  6. Specification, BOQ and estimate, then tendering, execution, site supervision, snag list and handover.
• Types of interiors: residential; workplace (open-plan, cellular, activity-based); retail and visual merchandising; hospitality (hotel, restaurant, café); healthcare; educational; institutional and civic; exhibition and set design; transport interiors.

Space planning, anthropometry and ergonomics

• Anthropometry is the measurement of the human body; ergonomics applies it so that reach, clearance, posture and sightlines suit the task. Indian body dimensions are smaller than the Western data in many textbooks - design to the Indian percentile data where available (5th percentile female for *reach*, 95th percentile male for *clearance*).
• Three dimension types: *structural* (static body), *functional* (dynamic, in motion - a 600 mm shoulder becomes a 750-900 mm walking corridor), *clearance* (space to pass, open a door, pull a chair).
• Working heights: kitchen counter 850-900 mm; washbasin 800-850 mm; dining and study table 750 mm; desk 725-750 mm; chair seat 400-450 mm; bar counter 1,050-1,100 mm with 750 mm stool.
• Depths and clearances: wardrobe 600 mm deep (hanging rod 1,050-1,100 mm clear for shirts, 1,500-1,600 mm for long garments); kitchen base unit 600 mm, wall unit 300-350 mm with 550-600 mm between counter and wall unit; corridor 900 mm minimum, 1,200 mm for two-way; dining chair pull-out 750-900 mm behind the table edge; bed side clearance 600 mm minimum.
• Standard furniture sizes: single bed 900-1,000 × 1,900 mm; double 1,500 × 1,900 mm; queen 1,525 × 2,000 mm; king 1,800 × 2,000 mm; sofa seat depth 550-600 mm, seat height 400-430 mm; dining allowance 600 mm width per diner.
• Circulation is typically 15-30% of carpet area depending on type (minimum in residences, maximum in retail and public lobbies). Dead-end corridors and crossing circulation through work zones are planning errors.
• Barrier-free requirements: the Rights of Persons with Disabilities Act, 2016 and the Harmonised Guidelines and Standards for Universal Accessibility in India (2021) govern - 900 mm clear door width, 1,500 × 1,500 mm wheelchair turning space, ramp slope not steeper than 1:12, grab bars 750-900 mm, switches 900-1,200 mm above floor, WC seat 450-480 mm high. The older IS recommendation for facilities for the physically handicapped has largely been superseded by these Guidelines; cite the Guidelines.
• Kitchen layouts: single-line, parallel (galley), L-shape, U-shape, island, peninsula; organised on the work triangle linking storage (fridge), washing (sink) and cooking (hob) - each leg about 1.2-2.7 m, perimeter about 4-8 m, with no major traffic crossing it.

Materials, finishes and furniture

• Flooring: vitrified tiles (IS 15622 for ceramic/vitrified tiles) - cheapest durable option, low water absorption in the fully vitrified class; natural stone (granite, marble, Kota, Jaisalmer); engineered and solid wood; laminate and vinyl (LVT/SPC) - fast, dry-fix, moisture-sensitive at joints; IPS and polished concrete; epoxy and PU for labs and industry; carpet and carpet tiles for acoustics in offices. Select on abrasion class, slip resistance (wet areas need a higher slip rating), water absorption and maintenance.
• Wall finishes: cement plaster with putty and acrylic emulsion (IS 15489 for interior acrylic emulsion paint), POP punning, textured paint, wallpaper, laminate and veneer panelling, tiles and stone cladding, fabric and acoustic panels, gypsum plasterboard partitions (IS 2095 for gypsum plaster boards) on GI studs.
• Boards and their IS numbers: plywood for general purposes IS 303 (MR - moisture resistant, BWR - boiling water resistant); marine/BWP plywood IS 710; block board IS 1659; particle board IS 3087; MDF/HDF IS 12406; decorative high-pressure laminate IS 2046; veneered decorative plywood IS 1328. Specify grade *and* thickness: 18 mm for carcass and shelves, 6 mm back, 0.8-1.0 mm laminate on exposed faces.
• Ceilings: gypsum board on GI framework (seamless, paintable, not for wet areas unless moisture-resistant board), calcium silicate board (wet/exterior soffits), mineral fibre grid tiles 600 × 600 mm (demountable, acoustic, service access), metal and wooden linear/baffle, stretch fabric, grid-and-channel for coffers and coves. Keep 250-400 mm service void for ducts, sprinklers and lights.
• Hardware and joinery: hinges (butt, concealed/European, soft-close), telescopic and under-mount drawer channels, lift-up and tandem systems, locks, handles, edge-banding; carcass joints - dowel, cam-and-dowel (knock-down), mortise-and-tenon, dovetail, biscuit, pocket-screw. Hardware life, not board, usually decides modular-furniture life.
• Furniture classification: by mobility (fixed/built-in versus loose), by construction (carcass, frame-and-panel, upholstered, modular, knock-down), by material (solid wood, engineered board, metal, plastic, cane/rattan, glass).
• Soft furnishing and textiles: curtains (fullness 2-2.5 times track length), blinds (roller, Roman, venetian), upholstery fabrics rated by rub count, rugs, cushions; used to tune acoustics, light and glare as well as appearance.
• Fire behaviour of finishes: NBC 2016 Part 4 (Fire and Life Safety) limits the use of combustible interior finishes on escape routes and in high-rise and assembly occupancies, and requires false-ceiling materials and cables with controlled flame spread. Use fire-retardant treated ply/MDF and class-rated panels where the occupancy demands it, and never block sprinklers or detectors with a ceiling design.

Colour, light and lighting design

• Colour theory: hue, value (lightness) and chroma/saturation; primary-secondary-tertiary on the colour wheel; warm (advancing, apparently smaller and cosier) versus cool (receding, apparently larger and calmer) colours.
• Schemes: monochromatic, analogous, complementary, split-complementary, triadic, achromatic. A common working rule is 60% dominant, 30% secondary, 10% accent.
• Colour in practice: light colours on ceilings raise the apparent height and improve light reflectance; high reflectance values (ceiling 0.7-0.8, walls 0.5-0.6, floor 0.2-0.3) directly raise the utilisation factor and cut lighting load; saturated colours are better as accents than as fields. Colour is specified by a maker's shade card plus an objective reference (NCS or RAL) so site cannot substitute.
• Light sources and qualities: LED has displaced CFL, fluorescent and halogen in Indian interiors. Specify luminous flux (lm), efficacy (lm/W), correlated colour temperature (2,700-3,000 K warm for homes and hospitality, 4,000 K neutral for offices, 5,000-6,500 K cool for workshops and task areas), CRI (Ra ≥ 80 generally, ≥ 90 for retail, art and clinical use), beam angle, dimmability and glare rating (UGR ≤ 19 for screen-based offices).
• Layers of light: ambient (general), task (desk, counter, mirror), accent (art, display), decorative (chandelier, pendant) and daylight. Good interiors use at least three layers with separate switching/dimming.
• Techniques: cove and valance lighting, wall-washing, grazing (texture), uplighting, downlighting, backlighting of onyx or acrylic, profile/linear LED in joinery, track lighting for flexible retail.
• Standards: recommended illuminance values and glare limits come from the National Lighting Code, SP 72 : 2010 and the IS 3646 series on interior illumination (parts of this series have been revised or withdrawn - quote the Lighting Code if unsure of a part number); NBC 2016 Part 8 Section 1 covers lighting and natural ventilation; ECSBC 2024 (successor to ECBC 2017; States adopt it in their own versions) for commercial and Eco-Niwas Samhita for residential buildings cap lighting power density (W/m²), pushing designs towards LED and controls.
• Daylight: orientation, window-to-floor-area ratio, light shelves, jaalis, courtyards, light-coloured reveals; NBC asks for openings for light and ventilation as a proportion of floor area for habitable rooms (check the current Part 8 clause and the local development control rules, which sometimes demand more).

Interior services, comfort and safety

• Acoustics: control at source, by separation and by absorption. Sound insulation (airborne) needs mass, isolation and sealing - a gypsum partition with staggered studs and insulation beats a single-leaf board; impact noise needs resilient underlay or floating floors. Reverberation is controlled by absorptive area - carpet, curtains, acoustic panels, perforated ceilings. NBC 2016 Part 8 Section 4 deals with acoustics, sound insulation and noise control; IS 1950 covers sound insulation of non-industrial buildings and IS 2526 the acoustical design of auditoria and conference halls.
• HVAC interface: diffuser and return-air positions in the RCP, access panels for VRF/FCU units, condensate slope, duct depth versus ceiling void, grille sizes coordinated with the ceiling grid. Rule-of-thumb cooling load in Indian residences is about 1 TR per 9-11 m² (100-120 sq ft) for normal exposure - a rule of thumb only; a proper load calculation is required for design.
• Electrical and low-voltage: circuit separation for lighting, power, air-conditioning; switch and socket heights; 5/16 A and modular plate layouts; earthing; MCB/RCCB in the DB; data, TV, intercom, access control, CCTV, home automation; conduits and cable routes resolved before false ceilings close. Follow NBC 2016 Part 8 Section 2 and the relevant IS wiring code; works are executed by a licensed electrical contractor.
• Plumbing and wet areas: keep WCs and kitchens near existing shafts; floor slope 1:80 to 1:100 to the trap; waterproofing in sunken slabs with a 150-300 mm upturn at walls; concealed CP fittings with access; vanity and mirror heights; ventilation or exhaust fans for internal toilets.
• Fire and life safety in fit-outs: maintain exit widths and travel distances, do not obstruct escape routes with furniture, keep sprinklers and detectors below false ceilings where required, use fire-rated doors and sealed service penetrations in compartment walls, provide extinguishers and signage. Interior fit-outs in malls, offices and restaurants need fire-department clearance and periodic NOC renewal as per the state fire service rules.
• Indoor environmental quality: low-VOC paints and adhesives, formaldehyde-controlled boards, fresh-air provision, plants; rating systems (IGBC, GRIHA, LEED India) award points for these, and office projects often specify them.

Practice, documentation and costing

• Drawing set: existing-condition plan, demolition plan, layout/furniture plan, flooring plan, reflected ceiling plan, electrical and lighting plan, plumbing plan, internal elevations, sections, joinery and detail drawings, finish schedule, door and window schedule, BOQ with specifications.
• Measurement and areas: carpet area is the net usable floor area within walls (defined for sale in the Real Estate (Regulation and Development) Act, 2016 as the net usable area excluding external walls, service shafts, balcony and open terrace, but including internal partition walls); built-up adds wall thickness and balconies; super built-up adds a share of common areas. Interior rates are usually quoted per sq ft of carpet area or per running foot of joinery.
• Estimating: detailed quantity × rate from the BOQ is the examinable method; approximate quotation at an all-in rate per sq ft is the market method. Common units - flooring and ceiling in m² or sq ft, joinery in sq ft of frontal/shutter area or running feet, painting in m², electrical in points.
• Taxes and contracts: interior work is a works contract supply - GST on such services is in the 18% slab under the 5/18/40 structure in force from 22 September 2025 (confirm the current rate and any item-wise notification before quoting a client). Contracts are item-rate, lump-sum or turnkey; retain 5-10% as retention until the defect-liability period ends.
• Professional regulation: only architects are statutorily registered, under the Architects Act, 1972, with the Council of Architecture - the title "interior designer" is not statutorily protected in India; the Institute of Indian Interior Designers (IIID) is the professional body. Statutory approvals for interior work (fire NOC, lift, society or building-management fit-out rules, structural clearance for any opening) are still mandatory.

Key points

• Interior design - organisation and detailing of enclosed space; decoration is only its surface layer.
• Anthropometry - body measurement data; use 5th percentile for reach, 95th percentile for clearance.
• Functional versus clearance dimension - 600 mm body width needs a 900 mm corridor and 1,500 mm turning circle.
• Work triangle - sink-hob-fridge legs 1.2-2.7 m each, perimeter about 4-8 m, traffic kept out.
• Standard heights - counter 850-900 mm, table 750 mm, seat 400-450 mm, wardrobe depth 600 mm.
• Circulation allowance - 15-30% of carpet area; residences low, public interiors high.
• Reflected ceiling plan - the mirror-image plan carrying ceiling, lights, diffusers and access panels.
• Plywood grades - MR and BWR in IS 303, BWP/marine in IS 710; laminate to IS 2046.
• Reflectance targets - ceiling 0.7-0.8, walls 0.5-0.6, floor 0.2-0.3; higher reflectance means fewer fittings.
• Lighting layers - ambient, task, accent, decorative, daylight, separately controlled.
• CCT and CRI - 2,700-3,000 K warm residential, 4,000 K office; Ra ≥ 80 general, ≥ 90 retail and clinical.
• UGR ≤ 19 - glare limit for screen-based workplaces.
• Lumen method - fittings N = E·A/(Φ·UF·MF); room index K sets the UF.
• Sabine's law - RT = 0.161 V/A; speech rooms 0.6-0.9 s, music halls 1.4-2.0 s.
• NBC 2016 - Part 4 fire and life safety limits interior finishes; Part 8 covers lighting, electrical, HVAC and acoustics.
• Accessibility - RPwD Act, 2016 with the 2021 Harmonised Guidelines; ramp 1:12, door 900 mm clear.
• Carpet area - net usable area as defined in RERA, 2016; the basis for most interior rates.
• GST - interior works contract falls in the 18% slab under the 5/18/40 structure effective 22 September 2025.

Formulas and examples

Example 1 - lumen method for an office. Room 9.0 × 6.0 m (A = 54 m²), floor-to-ceiling 3.0 m, luminaires surface-mounted at 2.70 m, work plane 0.75 m, required 300 lux. Fitting: 36 W LED panel, 3,600 lm. UF = 0.55, MF = 0.80.

• Hm = 2.70 − 0.75 = 1.95 m; K = 54 / ((9 + 6) × 1.95) = 54 / 29.25 = 1.85.
• N = (300 × 54) / (3,600 × 0.55 × 0.80) = 16,200 / 1,584 = 10.23 → provide 12 fittings in a 4 × 3 grid (spacing 2.25 m × 2.0 m, within a spacing-to-height ratio of about 1.2).
• Check achieved E = (3,600 × 12 × 0.55 × 0.80) / 54 = 19,008 / 54 = 352 lux - acceptable.
• Installed load = 12 × 36 = 432 W → 432/54 = 8.0 W/m², comfortably inside typical ECBC office lighting-power-density limits.

Example 2 - reverberation control in a conference room. Room 8.0 × 5.0 × 3.2 m → V = 128 m³. As built (vitrified floor, painted walls, gypsum ceiling) the total absorption at 500 Hz is 26 m² sabin.

• RT = 0.161 × 128 / 26 = 20.61 / 26 = 0.79 s - slightly live for speech.
• Add carpet over 40 m² of the floor (α = 0.30 replacing α = 0.03): added absorption = 40 × 0.27 = 10.8 m² sabin → A = 36.8 m² sabin.
• RT = 20.61 / 36.8 = 0.56 s - just below the 0.6-0.8 s target band for a conference room, so carpet over about 30 m² (A ≈ 34.1 m² sabin, RT ≈ 0.60 s) is the better stopping point; any further treatment would make the room too dead.

Example 3 - kitchen work triangle check. An L-shaped kitchen has sink-to-hob 1.8 m, hob-to-fridge 2.4 m, fridge-to-sink 2.1 m.

• Perimeter = 1.8 + 2.4 + 2.1 = 6.3 m, inside the 4-8 m band; each leg is between 1.2 and 2.7 m → layout is ergonomically sound. With a 600 mm counter and a 1,000 mm clear aisle the overall wet-zone width is 600 + 1,000 = 1,600 mm minimum.

Example 4 - paint quantity and cost for a living room. Room 5.0 × 4.0 m, height 3.0 m, openings 6 m².

• Wall area = perimeter 18.0 m × 3.0 m = 54 m²; net = 54 − 6 = 48 m². Ceiling = 20 m². Total = 68 m².
• Two coats of acrylic emulsion at 10 m²/litre/coat → (68 × 2)/10 = 13.6 litres; at ₹320/litre = ₹4,352, plus one coat of primer (68/10 = 6.8 litres at ₹200 = ₹1,360) → material ₹5,712, before labour and putty.

Example 5 - modular kitchen BOQ with GST. L-shape, base 14 running feet, wall units 10 running feet.

(Rates are indicative market rates and vary by city, brand and finish; GST slab as in force from 22 September 2025.)

Where it is used

• Turnkey and residential interiors: design-and-build firms and modular brands (Livspace, HomeLane, Sleek, Godrej Interio, Hettich and Hafele as hardware suppliers) employ designers for layout, 3D visualisation, BOQ and site coordination.
• Workplace fit-outs: IT and GCC offices in Bengaluru, Hyderabad, Pune and Gurugram - test-fit layouts, workstation density, acoustic zoning, IGBC/LEED documentation, with PMC firms supervising.
• Retail and hospitality: store rollouts for brands and QSR chains, hotel guest-room and banquet interiors - repeatable detail kits, visual merchandising, FF&E (furniture, fixtures and equipment) schedules.
• Real estate and institutional: developers' show flats and common-area fit-outs, CPWD and PWD interior tenders, bank branches, hospitals and campus interiors where accessibility and fire rules dominate.
• Specialist roles: lighting designer, acoustic consultant, FF&E procurement, 3D visualiser (SketchUp, 3ds Max, V-Ray, Enscape), BIM interior modeller (Revit), site engineer for interiors, estimation and billing engineer.
• Allied products: furniture and modular-kitchen manufacturing, tile and sanitaryware brands' design cells, paint companies' colour consultancy services.

Interview questions

• Q: What is the difference between interior design and interior decoration? A: Interior design plans and details the whole enclosed environment - layout, services, lighting, acoustics, safety and finishes; decoration only selects and arranges the surface and movable elements within an existing scheme.
• Q: Why do we use 5th percentile data for reach and 95th percentile for clearance? A: Reach must suit the smallest user or they cannot access it; clearance must suit the largest user or they cannot fit - designing both to the mean excludes half the population.
• Q: Explain the kitchen work triangle and its limits. A: It joins the storage, washing and cooking centres; each leg should be about 1.2-2.7 m and the perimeter about 4-8 m, with no through-traffic crossing it, so the cook's movement stays short.
• Q: How do you decide the number of light fittings for a room? A: By the lumen method - N = (E × A)/(Φ × UF × MF), with the utilisation factor read against the room index and surface reflectances, then check the spacing-to-height ratio for uniformity.
• Q: What is the difference between sound insulation and sound absorption? A: Insulation stops sound passing between spaces and needs mass, isolation and airtightness; absorption reduces reverberation within a space and needs porous or resonant surfaces - a soft acoustic panel absorbs but barely insulates.
• Q: Which plywood would you specify for a kitchen base unit and why? A: BWP/marine grade to IS 710, or at minimum BWR to IS 303, because the base units are exposed to repeated moisture; MR-grade ply belongs in dry bedroom joinery.
• Q: What is a reflected ceiling plan and what does it show? A: A mirror-image plan of the ceiling showing level changes, materials, luminaires, diffusers, sprinklers, detectors, speakers and access panels - it is the drawing that forces coordination between ceiling design and services.
• Q: How does colour affect the perceived size of a room? A: Light, cool, low-chroma colours reflect more light and recede, making a space feel larger; dark, warm, saturated colours advance and make it feel smaller and more enclosed, and a light ceiling raises the apparent height.
• Q: What interior-related fire-safety constraints apply to an office fit-out? A: NBC 2016 Part 4 limits combustible interior finishes, especially on escape routes; exit widths and travel distances must be kept clear, sprinklers and detectors must stay operative below false ceilings, fire-rated doors and sealed penetrations maintained, and a fire NOC obtained.
• Q: A client wants a wall removed to join two rooms. What do you check? A: Whether it is load-bearing or a shear wall, the structural engineer's clearance, services buried in it, the society or building fit-out rules, and whether removal affects fire compartmentation or the sanctioned plan.
• Q: What is carpet area under RERA and why does it matter to you? A: It is the net usable floor area within the apartment, excluding external walls, shafts, balcony and open terrace but including internal partition walls - most interior rates, furniture planning and client budgets are set per square foot of carpet area.
• Q: Is "interior designer" a statutorily registered title in India? A: No - the Architects Act, 1972 registers architects with the Council of Architecture, while interior designers are represented by a professional body (IIID) without statutory registration; the statutory approvals for the work itself still apply.

Common mistakes

• Quoting Western anthropometric tables as if they were Indian, and then designing counters and wall units too high for the user.
• Designing a layout from the furniture outwards and only later discovering the beam, duct, shaft or shear wall that makes it impossible.
• Forgetting clearance dimensions - a plan that fits the furniture but leaves no room to pull a chair, open a wardrobe shutter or pass a person.
• Using N = E·A/Φ without the utilisation and maintenance factors, or picking the UF without first computing the room index.
• Confusing absorption with insulation in acoustics answers, and mixing the 0.161 (SI) and 0.049 (FPS) constants in Sabine's formula.
• Specifying MR-grade board or unprotected MDF in kitchens and bathrooms, or omitting the IS grade from the specification so site substitutes a cheaper board.
• Designing a false ceiling without the service void, access panels and sprinkler/detector positions, then cutting it open after installation.
• Giving a lump-sum rate per sq ft in an estimating answer where a detailed quantity-and-rate BOQ was asked for, and omitting GST from the final client figure.

Related topics

Architectural design principles · Architectural drawing and presentation · Building materials and construction · Building services: lighting, HVAC and acoustics · Estimation, costing and specifications · Furniture design and ergonomics