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Can I build a 8' - 2x6" wall on top of a 3' - 6" CMU Wall ?

You will want to contact the local AHJ regarding their stance; however, it is my opinion that this would be permitted per R301.3 as you'd be less than 11'7" story height. This would be limited to a single story structure though, as if it is two story, the measurement is from floor to floor, which would put your story height at 12'6" or so. This is really not that different from a partial height basement wall with a framed wall on top of it (think stepped basement wall on a daylight basement home).

If required by the AHJ, you may have to hire an engineer to perform the structural design for lateral and gravity loading.

R301.3 Story Height
The wind and seismic provisions of this code shall apply to buildings with story heights not exceeding the following:
  1. For wood wall framing, the story height shall not exceed 11 feet 7 inches (3531 mm) and the laterally unsupported bearing wall stud height permitted by Table R602.3(5).
  2. For cold-formed steel wall framing, the story height shall be not more than 11 feet 7 inches (3531 mm) and the unsupported bearing wall stud height shall be not more than 10 feet (3048 mm).
    • Exception: An additional 8 feet (2438 mm) of bearing wall clear height is permitted for gable end walls.
  3. For insulating concrete form walls, the maximum story height shall not exceed 11 feet 7 inches (3531 mm) and the maximum unsupported wall height per story as permitted by Section R608 tables shall not exceed 10 feet (3048 mm).
  4. For structural insulated panel (SIP) walls, the story height shall be not greater than 11 feet 7 inches (3531 mm) and the bearing wall height per story as permitted by Section R610 tables shall not exceed 10 feet (3048 mm).
Individual walls or wall studs shall be permitted to exceed these limits as permitted by Chapter 6 provisions, provided that story heights are not exceeded. An engineered design shall be provided for the wall or wall framing members where the limits of Chapter 6 are exceeded. Where the story height limits of this section are exceeded, the design of the building, or the noncompliant portions thereof, to resist wind and seismic loads shall be in accordance with the International Building Code.

[RB] HEIGHT, STORY. The vertical distance from top to top of two successive tiers of beams or finished floor surfaces; and, for the topmost story, from the top of the floor finish to the top of the ceiling joists or, where there is not a ceiling, to the top of the roof rafters.​
 
The masonry wall needs to be able to resist the wind and other loads rom the wood framing above. This would require the involvement of a civil or structural engineer.. The footing of the masonry wall would likely need to be sider and the reinforcement in the masonry wall will likely need to be greater than for a free standing masonry wall.
 
Mark K said:
The masonry wall needs to be able to resist the wind and other loads rom the wood framing above. This would require the involvement of a civil or structural engineer.. The footing of the masonry wall would likely need to be sider and the reinforcement in the masonry wall will likely need to be greater than for a free standing masonry wall.
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The masonry wall could be designed prescriptively as a foundation wall per IRC Section 404, could it not?
 
Ty J. said:
The masonry wall could be designed prescriptively as a foundation wall per IRC Section 404, could it not?
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Maybe if it extended from top of footing to top of wall but not as an extension above the block as you have broken the structural plane. Is this a bearing wall?
Why not just frame it and apply veneer?
 
ADAguy said:
Maybe if it extended from top of footing to top of wall but not as an extension above the block as you have broken the structural plane.
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Where do you get this from?

Think of every daylight basement side wall wherein the foundation wall steps down with the grade and transitions to typical wood framing. Are you saying that this isn't permissible? See example image below.

Walk-out-Basement-Wall-Insulated-with-DOW-Foam-Board.jpg
 
The basement side wall was designed for significant lateral forces and if designed by and engineer considered the lateral loads from the stud wall. There is a problem in trying to extend the IBC provisions to situations that were not contemplated. This is a common problem with the way the IBC is used.
 
Mark K said:
The basement side wall was designed for significant lateral forces and if designed by and engineer considered the lateral loads from the stud wall. There is a problem in trying to extend the IBC provisions to situations that were not contemplated. This is a common problem with the way the IBC is used.
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The image was just an example; although, the IRC allows for up to 4-ft of unbalanced fill without an engineering design.

R404.4 Retaining Walls
Retaining walls that are not laterally supported at the top and that retain in excess of 48 inches (1219 mm) of unbalanced fill, or retaining walls exceeding 24 inches (610 mm) in height that resist lateral loads in addition to soil, shall be designed in accordance with accepted engineering practice to ensure stability against overturning, sliding, excessive foundation pressure and water uplift. Retaining walls shall be designed for a safety factor of 1.5 against lateral sliding and overturning. This section shall not apply to foundation walls supporting buildings.​

What the OP is asking, is to just carry the foundation wall upwards a few extra feet to building an impact panel or curb around the garage. Wood frame walls sit atop this foundation wall and have a stud length less than 10-ft (see IRC Table 602.3(5)).

IMHO, this is acceptable per the IRC; however, if using bonus room trusses, then it can be interpreted as violating the maximum story height per Section 301.3 as the floor-to-floor height exceed 11'7".
 
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