Many timber species in British forestry were selected for fast growth rate and imported from other regions within the temperate zone of the Northern Hemisphere. As these stands of trees have matured, the difference between UK grown and timber grown within the original provenance of the species has become apparent. One such example is Japanese larch (Larix kaempferi). The use of these home grown timbers presents challenges relating to growth rate and density. Japanese larch is a species of interest for diversifying softwood timber consumption in the UK, but sharply defined earlywood and latewood leads to several problems in machining and utilizing the timber. One option for enhancing timber quality is mild thermal modification. This paper presents a broad range of results from projects undertaken within Wales to develop and scale up the mild thermal modification process. The principal aims were to improve machinability and working properties of the timber in joinery applications. By comparing timber from treatment runs with residence times in the treatment zone which ranged from 1½ hours to 5 hours, an optimized protocol for furniture and joinery products was developed. Surface quality observations, bulk density and weight change were used to initially screen the efficacy of the treatment. The influence of the different thermal regimes on the level of modification are discussed. The efficacy of the treatment on regions of juvenile wood versus mature wood was also considered, as the difference in physical properties between juvenile and mature wood in untreated timber is significant and contributes to machining and working variability. The stability and machinability of large planks from the scale up were evaluated. The mild thermal modification shows potential for interior applications, and eradicates many issues commonly associated with fast grown larch such as resin pockets and springback of the latewood after machining.