GAF Home Additions & ADU Nationwide | Barrett & Sons Construction

GAF Home Additions & ADU: The Los Angeles Owner’s Guide

If you’re planning a GAF home addition or ADU in Los Angeles, the installed cost typically runs between $53,100 and $153,400 depending on square footage, foundation work, and finish level. Barrett & Sons Construction handles the full build, including GAF roofing systems that tie the new structure into your existing roofline so it looks like it was always there. We’re an independent Los Angeles contractor, not affiliated with GAF, but we’ve been installing their roofing and weatherproofing systems on additions across LA County for more than two decades. Most owners reach out after a previous contractor’s roof tie-in leaked during the first winter rain, or after they realize their ADU plans don’t account for Los Angeles hillside setback requirements.

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Key Takeaways:

• Installed cost for a GAF-roofed addition or ADU in Los Angeles: $53,100 to $153,400
• The roof tie-in is the #1 failure point on additions, especially on pre-1980 homes in Los Angeles
• Tom Barrett has been building additions and ADUs in Los Angeles since 1998
• Every job includes permits pulled, inspections signed, and written change orders before any price moves
• Free estimate: call (877) 610-1318 and ask for Tom

Which GAF Roof Systems Apply to Additions & ADUs

For Los Angeles home additions and ADUs, the GAF line that matters most is the Timberline series, particularly Timberline HDZ and Timberline UHDZ shingles, which carry GAF’s WindProven limited wind warranty and are rated for the Santa Ana wind gusts that hit the San Fernando Valley and foothill communities hard. A smaller ADU under 800 square feet can also work well with GAF Royal Sovereign, but we steer Los Angeles owners toward Timberline HDZ because the nail strip is wider and the sealant holds better when temperatures swing from 105° in Woodland Hills to 38° in the high desert. The critical compatibility gotcha: if your house was built before 1980 and still has the original roof decking, the new addition’s roof sheathing often doesn’t match the old spacing, and we have to re-sheet sections before the GAF underlayment goes down. GAF’s proprietary Deck-Armor and StormGuard underlayments don’t perform on compromised decking. We have also matched GAF shingle colors on additions where the main house got a new roof five or ten years earlier; GAF keeps color availability longer than most brands, but not forever, and a discontinued color means either reroofing the whole house or accepting a visible seam.

We also build additions and ADUs with KraftMaid cabinetry, Cambria or Silestone counters, and Andersen or Marvin windows when the job calls for them, so the whole structure carries name-brand materials you can look up yourself before you sign anything.

Signs Your Los Angeles Home Addition or ADU Plans Are Heading for Trouble

• Water staining at the roof tie-in within the first two years. This is usually faulty step flashing where the new roof meets the old wall, or the old roof deck was too soft to hold fasteners. We see this on Los Angeles additions built by crews who skipped the re-sheet.
• Doors that stick in summer and swing freely in winter. Los Angeles soil expands and contracts with moisture, and a foundation not engineered for the specific soil type on your lot will move. A proper addition starts with a soils report, not a guess.
• Hillside setbacks ignored until the inspector red-tags the job. Los Angeles hillside zoning has different setback rules than flat-lot zoning, and many owners don’t find out until the framing inspection fails. We pull permits and submit plans that account for your specific lot before the first shovel goes in.
• Mismatched rooflines that channel water into a valley. Some additions are framed to save time, not to shed water. The result is a valley where the new roofline meets the old, collecting debris and funneling water into the fascia.
• Secondary dwelling plans with no dedicated drainage plan. An ADU in Los Angeles changes your lot’s water flow, and without a drainage plan the rain that used to soak in now runs toward your foundation or your neighbor’s. We include drainage in every ADU scope, in writing.

Replacement Cost & What’s Included

For a full home addition or ADU in Los Angeles with a GAF roofing system, the installed range is $53,100 to $153,400. The low end covers a modest one-room addition up to about 400 square feet with basic finishes, a simple roof tie-in, and standard Los Angeles building permits. The high end covers an ADU up to 1,200 square feet with a full kitchen and bath, upgraded GAF underlayment, decking replacement on the existing roof, foundation work on unstable soil, and all inspections. What drives the price in Los Angeles specifically: hillside foundation engineering, existing roof deck condition, kitchen and bath finishes, and whether the LADBS requires additional plan review for your zone. Our pricing comes as a written line-item estimate before work begins, and any change to that number arrives as a written change order before it touches the job. We back the whole build with the 90-Day Done Right Promise, and the GAF shingle warranty runs 25 years to lifetime depending on the product line. Call (877) 610-1318 and we’ll walk the property and give you real numbers, not a phone quote.

DIY or Call Us?

The honest split: there’s almost nothing about a home addition or ADU in Los Angeles that a homeowner should DIY. The parts that seem approachable, like nailing down a roof deck or framing a wall, are the exact places where structural and safety mistakes compound. A roof tie-in done wrong leaks inside your walls where you won’t see it until the drywall is moldy. Foundation work done without a Los Angeles soils report can crack within two seasons. This is not a tile backsplash you can peel off and redo. If you already have GAF shingles and want to check the health of your existing roof before an addition, you can look for granule loss in your gutters and cracked sealant along the valleys, but even that only tells you it’s time to call someone who reads roofs for a living. Tom Barrett grew up in the San Fernando Valley and learned the trade at Los Angeles Trade-Technical College. We’ve been fixing other contractors’ addition mistakes since 1998. Call (877) 610-1318 and we’ll do it right the first time, or we’ll come back and make it right.

Frequently Asked Questions