The average fire truck weighs 36,000 to 60,000 pounds.

(it has to be true, I found it on the internet)

Turn the barn into an ADU???

Add a hydrant???

Add ADU to main house???

So is there a question? The OP has made a series of statements.

The first thing that I would do is find out what the actual law is and how it's enforced. You may be getting an inaccurate interpretation. Typically this kind of thing is enforced through a permitting process such as a Site Improvement Permit. Make sure that you understand the process of requesting a variance if indeed the stated code is accurate. There are probably local resources that can assist with questions on code and help steer you through the permitting process.

I think the OP is asking how much it will cost to improve a simple 270' gravel drive to a drive that will support a firetruck. I'm going to say 10k. +/- 5k :) The real answer is that it will vary by region for sure. Call a local excavator that does that kind of stuff and ask for an estimate. It doesn't have to be paved to support a firetruck.

The requirement of a new paved road was contained in a letter from the local Fire Department to my daughter. She had plans to build a new stand alone garage. Her plans were rejected because it would be the third building and she needed to improve the road to the barn.

"each building must be within 150 feet of a hydrant"

Small clarification: For projects I'm familiar with its been 150' from road access, and there are hydrants within 150' of that property line. Is it 150' from physical hydrant location to face of building? I would investigate the exact regulation that defines a "building" vs "accessory" structure - what are the square footage tiers and occupancy/uses? IIRC, you are in a horse property area - are there any properties with multiple barns and do they comply?

So some options I can think of:

- Driveway: Probably looking for 18' clear width (e.g. trees) and soil type, existing material, and elevation will determine amount of gravel (e.g. 12"+4"+4"), geo, or whether you can just top off existing. So the costs can be a huge swing from say $10K to $75K.

- Split the parcel into 2 properties.

- Convert part of the barn to an ADU.

- Add ADU on to house (just an openbreezeway roof might qualify it as connected).

- Park an RV in the barn.

- Last option, and would never recommend to a private residential property, is extend water main to hydrant on property.

we deal with issues like this once and a while and adding fire sprinklers mitigates the need to have access large enough for a fire truck.

sometimes its one house with a ling narrow driveway and other times ita a whole city block thats too narrow to turn around.

its not a cheap option but a lot cheaper than adding a large underground main and hydrant.