Room additions & extensions in 4e Arrondissement (Marais Sud / Île Saint-Louis)
4e Arrondissement (Marais Sud / Île Saint-Louis) is 4e arr. — Hôtel-de-Ville's 17th-century marais submarket. Île Saint-Louis is one of the few Paris quartiers where the original 1614-1664 cadastral grid is fully preserved — Permis de Construire reviewers cross-reference your dossier against the original Le Vau-era plot lines and reject any extension that crosses pre-Haussmann lot boundaries.
What a room additions & extensions project looks like here
Île Saint-Louis is one of the few Paris quartiers where the original 1614-1664 cadastral grid is fully preserved — Permis de Construire reviewers cross-reference your dossier against the original Le Vau-era plot lines and reject any extension that crosses pre-Haussmann lot boundaries.
Because the 4e backs onto the Seine on three sides, basement waterproofing scope on rive-droite buildings is non-trivial — any cave conversion to habitable space triggers PSMV review of the cuvelage system and a hydrogeological étude.
Paris extensions are rare in the historic core — typically attic / combles conversion or rooftop addition (surélévation). PLU governs maximum hauteur plafond, Permis de Construire required, ABF veto on PSMV-listed faubourgs. In 4e Arrondissement (Marais Sud / Île Saint-Louis) specifically, 17th-century marais stock means room additions & extensions scope is shaped by the neighborhood's dominant construction typology. Baily's Paris scoping flow factors psmv marais and île saint-louis: zppaup overlay + 1614 grid preservation into the estimate before a contractor is involved.
Start your 4e Arrondissement (Marais Sud / Île Saint-Louis) scope — Baily asks the right questions.
Pre-seeded for room additions & extensions in 4e Arrondissement (Marais Sud / Île Saint-Louis). Mention your 40-200 m², your timeline, and any known constraints — Baily factors the mairie du 4e + mairie de paris (du) + abf + psmv marais review queue into the scope.
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Who is Baily?
Baily is named after Francis Baily — an English stockbroker who retired at 51, became an astronomer, and in 1836 described something on the edge of a solar eclipse that nobody had properly articulated before: a string of bright beads of sunlight breaking through the valleys along the moon’s rim.
He wasn’t the first to see them. Edmond Halley saw them in 1715 and barely noticed. Baily’s contribution was clarity — describing exactly what was happening, in plain language, so vividly that the whole field of astronomy paid attention. The phenomenon is still called Baily’s beads.
That’s what we wanted our AI to do. Every inbound call and text has signal in it — a homeowner’s real question, a timeline, a budget, a hesitation that means “yes but.” Baily listens to every one, 24/7, and finds the beads of light.
Baily was a businessman before he was a scientist. That’s our vibe too.
Questions LA homeowners actually ask
4e Arrondissement (Marais Sud / Île Saint-Louis) room additions & extensions projects typically run $75K–$385K. 4e Arrondissement (Marais Sud / Île Saint-Louis)'s 17th-century marais stock, combined with psmv marais — full coverage, puts most mid-complexity projects in the $230K range. Baily scopes the exact band once you describe the work.
Nearest neighborhoods
Same service, adjacent Paris submarkets.