Room additions & extensions in Île Saint-Louis
Île Saint-Louis is 4e arr.'s 1614-1664 le vau planned-grid + 17-18th c. hôtels particuliers submarket. Île Saint-Louis was developed 1614-1664 as a planned grid by Le Vau and his contemporaries — the original parcel boundaries are preserved in current cadastre, and any extension that crosses an original lot line is routinely rejected at first review regardless of current PLU permissions.
What a room additions & extensions project looks like here
Île Saint-Louis was developed 1614-1664 as a planned grid by Le Vau and his contemporaries — the original parcel boundaries are preserved in current cadastre, and any extension that crosses an original lot line is routinely rejected at first review regardless of current PLU permissions.
Because the island sits on Seine alluvium with shallow water table, basement waterproofing (cuvelage) is non-trivial — a hydrogeology étude is required for any cellar conversion, adding 8-14 weeks to the dossier.
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 Île Saint-Louis specifically, 1614-1664 le vau planned-grid + 17-18th c. hôtels particuliers stock means room additions & extensions scope is shaped by the neighborhood's dominant construction typology. Baily's Paris scoping flow factors 1614 le vau plan and abf avis conforme into the estimate before a contractor is involved.
Start your Île Saint-Louis scope — Baily asks the right questions.
Pre-seeded for room additions & extensions in Île Saint-Louis. Mention your 60-300 m², your timeline, and any known constraints — Baily factors the mairie du 4e + du + abf conforme + psmv marais (extended) 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
Île Saint-Louis room additions & extensions projects typically run $75K–$385K. Île Saint-Louis's 1614-1664 le vau planned-grid + 17-18th c. hôtels particuliers stock, combined with 1614 le vau plan — original cadastral grid preservation, 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.