I propose you a simple and eclectic style, with some formal arranges, keeping your current floors marble and wooden. (for this point it's pretty important to have specially care with the areas when walls are gonna be retire). Also keeping the current furnitures, those could be restore and use to combine with the new ones.
My propose is pretty simple, the idea is you don't have to invest a lot of money in every detail, because there are some items that have a high cost in bathrooms like the shower tub for the main bathroom and the rest of items in bathrooms, this space of the house is pretty important for having a confortable and nice place for enjoying, as the kitchen is too, for that reason in the kitchen in main apartment i propose a large space semi open to the living room, we're gonna integrate a green element to give more life to this space, it's a green wall next to the wall of the kitchen window, you can cultivate some herbs or spices here (you can get these in the market easily). I propose to have an accent wall betwen the living area and the kitchen with perforated bricks, this wall divide the space being light at the same time so in some way it can integrate both spaces. For new walls you can use dry wall is an easy and cheap way to build new walls in interiors.
For the second apartment i don't detail to much because i think it could be simple for the tenants can furnish as they wish, we can only focus the attention of the open kitchen area and room with the same color used in the kitchen of the main apartment, the kitchen is openly designed and the furniture can be seen in the image of kitchen furniture. The bathroom can use the same elements as the bathroom 2 of the main apartment. The floor in this case could be laminated wood it's easy to install, cheap but good look.
About the walls we need to use trend's colors only in some walls (where i show in the renders) the rest of the walls just need to be restore and painting in white in order to have a style free of visual recharging items but not minimalist neither, it's just an eclectic, warm and light space.
Note: in renders i don't use the floor exactly as it is, but the idea is to maintain all the floors of marble and wood.
Except in bathrooms where the floor and walls change completely.
In the dwg file you can find the architectural plan, the wall plan to be demolished, and the lighting plan.
