Start with the certificate of occupancy, not with a contractor.

Before anyone opens a wall, get the town's copy of your certificate of occupancy and read it against the house you are standing in. It lists what Islip has on record. Anything in the house that is not on that list is work you have to account for, and the list is usually shorter than owners expect.

The items that go missing most often are a finished basement, a rear addition, a garage turned into a bedroom, a deck, a shed, an egress window cut into a foundation wall, and a second kitchen that appeared downstairs at some point in the 1990s.

Then list every one of them honestly. The town's own instruction is to read the certificate first and leave off whatever is already on it, and the application warns that more fees may apply if further unpermitted structures turn up during inspection. Guessing low only moves the discovery to the worst moment for it, with an inspector already standing in the house.

Four years old is the line that decides which application you file.

Islip splits after-the-fact work at four years. Anything built without a permit more than four years ago goes on a residential compliance permit. Anything newer than that is an ordinary building permit application. An accessory apartment is the exception, and it needs a compliance permit no matter how old it is.

The compliance permit application published by the Islip Building Division also covers permits that ran past their maximum renewals, which is a common and different situation: somebody did pull a permit, then never got it inspected and closed out.

One more detail before you start. The compliance permit expires a year from the date it is issued and cannot be renewed. You cannot file it, get busy, and pick it up again later.

A current survey is almost always the long pole.

The survey sets your calendar, so order it first. Islip wants one less than a year old, drawn to scale and signed and sealed by a New York State licensed surveyor, showing every structure on the lot with its dimensions, its ridge height and its distance to each property line.

The survey in your closing folder does not qualify, and on a lot that has collected a deck, a shed and a patio since then it is not accurate either. The town also wants lot area, property line bearings and distances, the tie distance to the nearest street corner, and the heights of perimeter fences and walls.

Order it early for a second reason. Those distances to the property line are what decide whether this is paperwork or a zoning fight, and you want that answer before you file anything.

Floor plans, and when hand drawn is good enough.

Homeowners assume this step needs an architect. Usually it does not. For an interior alteration, an addition or an accessory building, Islip asks for floor plans of the whole structure including the basement, and those plans may be hand drawn as long as they are neat and legible. The town suggests lined paper and a ruler.

What they show is specific: every space labeled for what it is, doors and windows located, exterior and room dimensions given, and the ceiling height of any basement. Measure that height before you go further. It decides whether the space downstairs can be habitable at all, and no paperwork raises a girder. Our piece on finishing a basement on Long Island covers what else that space has to satisfy.

The exception is the accessory apartment. If that work is under four years old, the plans must be prepared, signed and sealed by a licensed architect or engineer.

The zoning denial is what turns a few months into a year.

Paperwork problems get solved in weeks. A zoning problem does not. If what was built does not comply with the Town of Islip Zoning Code, the Building Division issues a denial letter and the file stops moving until you either change what is there or win a variance at the Zoning Board of Appeals.

Price that into the timeline before you list the house. A rear addition crowding the side setback, or a garage conversion that drops the lot below its required off-street parking, reads fine on the finished house and fails on the survey. The appeals board meets on its own calendar, and no contractor moves that calendar for you. Our walkthrough of garage conversions in Islip and Babylon goes deeper on the parking rule.

Then the walls come back open.

Once the permit issues, this stops being paperwork. You schedule building and plumbing inspections with the town's building records office, and an inspector has to see what is behind the finish. On a basement a previous owner closed up neatly, that means cutting the drywall back open at the framing, the connections and the fasteners.

Expect a punch list rather than a signature. The inspector says what further documentation, certifications or corrective work are needed, and whether that work needs its own permit before it starts. Electrical work needs a certificate from an inspection agency the town recognizes, which is a call to make early. Plumbing done without a permit more than four years ago gets added to the same application and inspected with it.

A licensed contractor earns the fee here, because opening a finished wall for an inspector and closing it back up properly is real carpentry. If structural work turns up, we bring in a licensed engineer. Call (631) 983-8079 and we will walk it with you before anything gets cut.

Suffolk County usually has a say, and it is usually about the cesspool.

Islip will not issue the final certificate until Suffolk County signs off, wherever county approval was required for what got built. The county's Office of Wastewater Management reviews what a house discharges, so work that changed the bedroom count or added a bathroom lands on its desk whether or not anyone told it at the time.

The county publishes the forms for exactly this on its residential permits page. Additions, remodeling and conversions go on Form WWM-057, with a separate certification form for existing sewage disposal and water supply at a single family residence.

On the south shore this is where a legalization stops being a paper exercise. Plenty of post-war capes and 1950s ranches in Bay Shore and Islip still run on their original system, sized for the house as built. A previous owner who put two bedrooms downstairs raised the count without touching the system, and this review is what finds it.

Flood zone and wetlands: the south shore complication.

Two further reviews catch properties south of Montauk Highway in particular. If any part of the lot falls inside the federally mapped floodplain, the work has to meet those regulations and more requirements attach. If the property sits in a wetlands area, a separate wetlands and watercourses permit comes from the Planning Division.

Planning also decides during that review whether state environmental approval is needed on top of the town's. Neither is rare near the Great South Bay, so find out what the lot sits in before you commit to a schedule.

What drives the cost, and when to start.

Nobody prices a legalization from a description, because the number is set by what an inspector finds once the wall is open. What moves it is how much was built, how much has to be uncovered, whether the electrical work can be certified as it sits, and whether a variance or a county review enters it.

On timing, start now. The survey, the records request and the filing all move through the fall and winter with no crew on site, and the opening and closing work is comfortable indoor work in January. If the plan is to list in spring, the paperwork should already be moving. For the rest of that sequence, see our notes on which repairs to do before selling, our overview of what each town wants before you start, and what we handle on home additions.