After a fire has ignited and steady burning is established, the fire enters a stage of growth.

In Figure 6, the fire has ignited behind the couch, possibly due to an electrical outlet or extension cord. The fire has established itself and is now growing up the wall, fuelled by the fabric and foam in the couch. In the early stage of the fire, the HRR and physical size of the fire are essential parameters, as those determine how fires initiating from different fuels will grow and spread.

Typically, when a fire begins, there is only one fuel item that has ignited and is burning. The driving force for the initial fire growth is primarily flame spread across this item. Thus, the characteristics of the original item govern fire size in the early stages of growth.

Subsequently, the fire size increases as flames spread to nearby fuels, either by direct flame contact, or by radiant ignition. As previously discussed, the potential for a fire to spread will depend on important factors such as fuel orientation, fuel arrangement and the exposed surface area (or the ratio of surface area-to-mass of fuel). The combined HRR of the initial fuel(s) are important factors determining the rate of fire growth. Finally, the distance between different fuels is important. The closer flammable items are to each other, the greater the potential for multiple items being ignited earlier in the fire.

In the fire growth period, rapid flame spread across any fuel item contributes to a larger burning area (area of the fire), which in turn increases the HRR as more and more fuel becomes involved in the oxidation reactions. As the fire continues to grow and impact its surroundings through combined heat transfer via convection, conduction and radiation, temperatures of all items within the enclosure continue to increase. When the growing fire generates sufficient heat to overcome the combination of heat lost to the walls and to heat flowing out of the enclosure in the escaping smoke, an energy feedback loop can lead to a thermal imbalance. The fire HRR grows to a maximum, approximately steady value. The end result of the thermal imbalance is that the fire undergoes a transition, termed "flashover," and enters the fully developed stage.