SDP, Emprise au Sol, SHON, SHAB: Four Numbers, One Building, and Why They're Never the Same
You ask someone how big their house is. They say 120 m². You measure it yourself. You get 142 m². You calculate the SDP for the planning application. You get 97 m². The thermal engineer asks for the SHAB. You get 86 m².
Same building. Four different numbers. All of them correct.
If that sounds like France being France, it isn't, or at least not entirely. Each of these surfaces measures something specific for a specific purpose. Once you understand what each one is actually counting, the numbers stop being contradictory and start making sense.
By the end of this guide you'll know what each surface means, how it's calculated, where it applies, and why getting them mixed up will cost you time, money, or both.
Why These Numbers Matter: The Thresholds
Before getting into the formulas, it's worth understanding what's at stake. Three dimensions determine your administrative path: surface de plancher (SDP), emprise au sol, and height.
- Under 5 m² SDP or emprise au sol, under 12 m tall: no formality. Garden shed territory.
- 5 m² to 20 m²: déclaration préalable. Lighter-touch, faster.
- Over 20 m²: full permis de construire. Extensions in a PLU zone urbaine can go up to 40 m² on a DP, but the moment the total SDP of the building after works exceeds certain thresholds, the rules change.
The one that catches everyone: if the total SDP after works exceeds 150 m², you're legally required to engage an architect. Not the new area. The total. An existing 130 m² house extended by 25 m² hits 155 m² total. Architect required. 150 m² sounds like a lot. It isn't, once you count what's already there.
These thresholds are why the SDP calculation matters. Get it wrong by a few square metres in the wrong direction and you've either undersold your project to the mairie or triggered obligations you hadn't budgeted for.
Surface de Plancher (SDP): The Planning Surface
The SDP is the one that drives everything in a planning application. It's the figure that goes on the cerfa form, determines your threshold, and governs the 150 m² architect rule.
The definition: the sum of all enclosed and covered floor areas with a ceiling height strictly above 1.80 m, measured from the interior face of the walls.
That interior measurement is worth dwelling on. SDP is measured from the inside of the structure: the interior face of your blockwork, your timber frame, your concrete walls. The thickness of your exterior walls doesn't count. Neither does your external insulation. If you're building in ossature bois with 300 mm of wood fibre on the outside, none of that eats into your SDP. The state rewards you for insulating from the outside, even if that wasn't quite the intention.
The Deductions
The number on your planning form will almost always be lower than your gross floor area. That's not an error, it's the deductions working correctly.
Spaces under 1.80 m ceiling height. Any area where the clearance drops to 1.80 m or below is out. The classic case is combles aménagés with a sloped roof: you measure along the floor, find the exact line where the underside of the rafters hits 1.80 m, and everything beyond that line towards the eaves doesn't count. The 1.80 m rule applies everywhere, not just attics. Which brings us to the under-stairs question.
The under-stairs battleground. This is where self-builders and first-time applicants consistently come unstuck. The space physically beneath a staircase (a cupboard, a toilet, a storage nook) follows the same 1.80 m rule as everything else. You map the exact line where the underside of the staircase drops to 1.80 m above the finished floor. Everything on the "can stand up" side of that line counts toward SDP. Everything on the "can't stand up" side doesn't. If you're squeezing a WC under the stairs, you only declare the floor area where a full-grown adult can stand without rearranging their skull.
Staircases and lift shafts. The void (trémie) created by a staircase is excluded on the upper floors. On the ground floor, the footprint of the staircase counts: you're standing on something. On the first floor, the open void above those stairs is deducted. You don't pay a surface tax on empty air.
Car parking. Garages, parking spaces, internal access ramps, manoeuvring areas: all excluded, entirely. A garage contributes zero to your SDP regardless of how big it is or how well-finished it is. This is the biggest single deduction for most maisons individuelles.
Non-habitable roof spaces. Attic volume that genuinely can't be converted, because the structural trusses block the space (the classic W-truss industrial fermette where you can only crawl), or because the floor can't take living loads. If it's genuinely inaccessible and unconvertible, it's out.
The Traps at the Bottom of the Official Form
The official SDP calculation worksheet (fiche d'aide pour le calcul de la surface de plancher, article R.111-22) has a section at the bottom with three additional deduction lines, each marked with a small asterisk. The note reads: ces déductions ne concernent pas l'habitat individuel.
These deductions don't apply to houses. Zero on all three lines, every time, no exceptions:
Locaux techniques: boiler rooms and utility spaces for collective buildings. Your heat pump room, your utility room, your plant space: if it has a ceiling height above 1.80 m, it counts toward your SDP. Can't deduct it.
Caves ou celliers: basements and storage rooms accessed via shared communal corridors in apartment buildings. Your private basement or wine cellar in a house: if the ceiling is above 1.80 m, it counts. Can't deduct it.
The 10% deduction: a flat reduction given to apartment buildings with internal common areas to compensate for insulation thickness in shared corridors. Houses get 0%. Not a rounding error. Zero.
From ten years on sites and at the drawing board: the locaux techniques trap is the one that catches the most people. Someone builds a beautiful utility room, measures everything, files the cerfa, and either genuinely doesn't know the room counts toward SDP, or quietly hopes no one checks. The mairie checks. Budget the utility room into your SDP from day one.
Emprise au Sol: The Footprint
Where SDP counts interior floor area, emprise au sol counts the physical occupation of the land. Think of it as a vertical projection: the shadow the building would cast if the sun were directly overhead. Any part of the ground covered by the building or its structure counts.
Unlike SDP, emprise au sol includes the thickness of exterior walls. It's measured from the outside.
What counts:
Anything with a roof or cover over it that's structurally attached to or part of the building. The main structure obviously. Garages (which have zero SDP but full emprise au sol). Carports: open-sided, no walls, zero SDP because there's nothing enclosed, but the covered footprint counts as emprise au sol. Overhangs and cantilevers where an upper floor extends beyond the ground floor. Covered porches and entrance canopies structurally attached to the building.
What doesn't count:
An open terrace built at ground level directly on natural terrain: zero emprise au sol, zero SDP. The moment that terrace is raised on a structure (foundations, pillars, blockwork walls), it starts counting. A detached garden shed that's not touching the main building has its own separate emprise au sol, not added to the house's.
Why it matters beyond planning: many PLUs include a coefficient d'emprise au sol: a maximum percentage of the plot that can be covered by buildings. It's not universal, but where it exists, exceeding it means going back to the drawing board regardless of what your SDP says. Check your PLU early.
SHON and SHOB: The Old System You'll Still Encounter
Surface hors œuvre brute (SHOB) and surface hors œuvre nette (SHON) were abolished on 1 March 2012. They do not exist in modern planning applications. Do not write either of them on a cerfa form.
They haven't quite disappeared though.
SHOB was the gross total floor area of all levels, measured from the exterior face of the walls. Everything included: balconies, parking, technical spaces, the lot. No deductions.
SHON was SHOB minus a defined list: areas under 1.80 m, parking, deep balconies and terraces, technical spaces, and a fixed reduction for insulation thickness. What remained was the net planning surface.
The critical difference from SDP: both SHOB and SHON were measured from the exterior of the walls. SDP is measured from the interior. This matters when you're trying to convert between the old and new systems: they don't translate directly, so as a rough rule of thumb, an old SHON figure converted to SDP typically drops by around 10-15%, because the exterior wall thickness disappears from the calculation. But it's a rough guide, not a formula.
Where you'll still encounter them: older property deeds, building permissions from before 2012, some older PLU documents that haven't been fully updated, and occasionally in notarial documents and bank valuations. If you're buying an older property and planning to extend, the original permission may reference a remaining SHON capacity. You'll need to understand what that means in current SDP terms before you can work out what headroom you actually have.
SHAB: The Rental and Residential Surface
Surface habitable (SHAB) has nothing to do with planning. It doesn't appear on any cerfa form, it doesn't trigger any planning threshold, and it's not defined anywhere in the code de l'urbanisme.
It's defined in the code de la construction et de l'habitation (article R.111-2) and it governs the residential and rental world: unfurnished lease agreements under the loi Boutin, property descriptions, and the loi Carrez for property sales (technically a slightly different measurement, but often used interchangeably in practice).
What it measures: the floor area of habitable spaces, measured from the interior face of the walls, excluding walls, partitions, steps and stairwells, ducts, door and window reveals, and any space with a ceiling height below 1.80 m.
In practice, SHAB is almost always lower than SDP. A house with an SDP of 110 m² might have a SHAB of 93 m². The gap represents spaces that are enclosed and covered (so they count toward SDP) but not considered habitable: a generously-sized entrance hall measured conservatively, a utility room, storage spaces.
Why accuracy matters: if you're renting out a property, the SHAB stated on the lease must be correct. If you underquote by more than 5%, the tenant has the right to a proportional rent reduction. This is not a theoretical risk. It gets exercised.
The Four Surfaces: A Practical Summary
Same house, four different questions:
SDP answers: how much floor area does this project create for planning purposes? Emprise au sol answers: how much of the plot does this building cover? SHON (old) answered: how much net floor area does this building have for the pre-2012 planning system? SHAB answers: how much of this building is genuinely habitable for rental and residential purposes?
Use the wrong one in the wrong context and you're either misrepresenting your project to the planning authority, miscalculating your remaining headroom under an old permission, or exposing yourself to a tenant dispute.
Know which number you need before you start measuring. Everything else follows from that.
Official Resources
📄 Fiche de calcul — Surface de plancher (article R.111-22 du code de l'urbanisme)
📄 Article R.111-2 du code de la construction et de l'habitation — Surface habitable