Living Area Calculation

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I am working on an ADU and the county requirements say it needs to be 900 square feet or less. On one hand, the county code says:

1. The ADU cannot exceed 50% of the square footage of the habitable area of primary residence or 900 square feet, whichever is smaller

I have 3 questions:  Is "Habitable Area" determined on dimensions from Interior or Exterior walls when it comes to the 900 square feet requirement? Does this vary city to city or is there a code standard?

My current plan has a total of 900 square feet. I also need to know if this total is based on a measurement from Interior walls or Exterior walls? How do I confirm? Is there a default setting where you set this to factor for living area calculations?

Is there a standard in the IBC which states that the living area is always calculated on Interiors or Exterior walls?

I will need to reduce my footprint if I find the requirement is based on exterior walls dimensions.

Any experience with this code situation and knowledge on how to confirm this calc within my settings would be greatly appreciated.

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I think in the definitions section (of the building code) it describes where the calculation occurs.

On a related note: it would be nice if there was some graphical way that CA showed the area that it is calculating. - and maybe there is, I'm a new user, so its possible I just haven't found it yet.

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I have done several of these in California and the answer is each city has their own set of rules and they all differ.. As for square footage  all of the cities I work for use the exterior walls as a typical method of figuring out the square footage all around. you can use the finish schedule to automatically show square footage if you add that column in the schedule.--Be aware that chief splits the wall between the living area and the garage, everyone else doesn't..

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2. I would not rely on ca calculations....

2. Draw a rectangular polyline or use a footprint to create the polyline .... Put owner.area.round(1) macro as a label it will show you the sq ft. Adjust to what the building department tells you.

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Click on any area just outside an exterior wall.  You may have to tab in a couple of times but it will eventually highlight the building and deck perimeter.  You can then click on "make living area polyline".  This will show a crosshatched box of what you are looking for.

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ANSI Z765 2013

See 2017 post.

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many different "official" ways of doing the calculation

depends on who needs the number(s)

FHA/HUD Handbook 4150.2

Fannie Mae Selling Guide October 2013

Employee Relocation Council (ERC) Appraisal Guide

Thomas, David Hampton. The American Measurement Standard: Calculating Residential Square Footage.

Institute of Housing Technologies, 2009.

Thomas, David Hampton. Size Matters! Solving the Square Footage Puzzle--Appraiser's Edition. 2009. US Department of Housing and Urban Development. Valuation Analysis for Single Family One- to FourUnit Dwellings, Handbook 4150.2, Sect. 3-3A.

Wright, Larry T. Review of Size Matters! Measuring and Calculating Residential Square Footage. The Appraisal Journal (Summer 2008): 285-286.

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20 hours ago, DRAWZILLA said:

Be aware that chief splits the wall between the living area and the garage, everyone else doesn't..

1

Perry - CA fixed this a few versions ago.  I just checked to be safe, and it doesn't split the Sq Ft between the common wall between the garage and house anymore.

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Funny ,I still get discrepancies between using the p-line method and the finish schedule method, they are not the same. When I use the p-line square footage method and split the house to garage  wall, ( and I always use both ) as a check it will come close to being equal.   I use p-lines for site plans and property calculation's. I have never had them match exactly and have thought of removing the s.f. column from the finish schedule.

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Maybe CA can add an approximate symbol (~) to the s.f. column.

Many years ago I once built a custom home for an engineer.  He tried to deduct HIS calculated square footage from the plans stated s.f. from his final billing.  His reasoning being that "Total Living Area" can only include the usable, living space in a room and surely can NOT include the space under walls, closets, stairs, cabinets, utilities, etc.

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On 1/19/2019 at 2:41 PM, javatom said:

Click on any area just outside an exterior wall.  You may have to tab in a couple of times but it will eventually highlight the building and deck perimeter.  You can then click on "make living area polyline﻿".  This will show a crosshatched box of what you are looking for.

Thank you. Based on what you have shown me, does this confirm the living area calculation of 900 square feet is based on outside walls? See the image I have uploaded for reference. Thanks in advance.

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Open the living area polyline. You will see the area it encloses.

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2 hours ago, wpdesigninc said:

Thank you. Based on what you have shown me, does this confirm the living area calculation of 900 square feet is based on outside walls? See the image I have uploaded for reference. Thanks in advance.

As others have mentioned before, check local codes, BUT, realistically, 'living area' is any space where you can physically stand in, where appliances, cabinets, etc. are removed.

Bottom line: living area = inside walls from my perspective.

CA can show:

• standard area: outside walls
• interior area: inside walls, = living area
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Bottom line:

it all depends on who's asking ...

Lew

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• 1 month later...

Hei guys) Need to make construction calculation for my house project... have never been using anything like that and need advice in the usage... Friends proposed to use the next site concalc.org and said that it is one of the best on today's building market. Have any of you worked with it? And if yes... what was your experience?

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Here (NY) it is always to the interior for habitable space, to the exterior for taxable space.
Also, the dimensions here are to net space, so a room that is framed at 12'-1", with 1/2" Drywall will be a net of 12'.
If the dimensions were to the outside of the wall, there would be no way to account for a wall that is common to two rooms, as each one would claim the common wall for its own.
Another thing, room dimensions, i.e. floor space is also relative to the structure above, so if you have a sloping ceiling above the room, any area of the room which has a ceiling height of less than 5' (that's what it is here in NY) does not count for habitable area.

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