**Favor 12" OC joists over additional rows of blocking, and keep the planned blocking.** For otherwise identical framing, closer joist spacing is the more dependable way to improve firmness.

They help in different ways:

- **Closer joist spacing** reduces the decking’s span between supports and reduces the load carried by each joist under a distributed load.
- **Blocking** keeps joists upright and can help neighboring joists share a concentrated load. Its effectiveness depends on the connections. It does not create a supported midpoint or shorten the joists’ vertical bending span. Research on timber floors describes these distinct effects on longitudinal and transverse stiffness. [Timber-floor vibration study](https://pmc.ncbi.nlm.nih.gov/articles/PMC10774337/)

Assuming your joists run outward from a house ledger to a beam, your layout has roughly **12 feet between supports, followed by a 2-foot cantilever**. Joist depth matters greatly over that span. Deeper joists—or an additional properly supported beam that shortens the span—can deliver a larger improvement than changing spacing alone.

For perspective, a simple beam calculation gives **25% less joist deflection under the same uniformly distributed deck load** when spacing changes from 16" to 12", with joist size and span unchanged. That does **not** mean 25% less perceived bounce under someone’s footsteps; decking, load sharing, and beam movement also affect the result.

For your goal, I would have the designer:

- Compare deeper joists with the proposed joists at 12" OC.
- Check beam stiffness and spacing between posts, since beam movement affects the whole deck.
- Keep the midpoint blocking and detail blocking at the cantilever’s supporting beam. AWC’s DCA 6 calls for blocking there and treats the main span and cantilever as separate checks. [AWC deck construction guide, Figure 6 and Table 2](https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf)

**12" OC is my choice between your two options, but joist size and beam design determine whether it will feel solid.** What joist size, lumber species/grade, decking material, and beam/post layout are planned?