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Chandani You have a great space with amazing light! To delineate the two spaces, I think area rugs are the best option. Two large rugs to anchor the spaces will define the spaces and warm things up. Something tall at the end of the credenza, large plant?, might also visually separate the space. As far as the expanse of wall, based on your furniture, you could do a collection of art- maybe 3 large canvases over the credenza and 2 in the dining are. A collection of photographs in black/dark brown frames with crisp white mats could also work.

foxscully I love this space! I'm going to expand on the comment from Chandani by suggesting that the plant that you put at the end of the credenza be a dracaena (only if you don't own a dog), or a few snake plants in a box on the credenza, something like this perhaps:
http://www.modernhomeidea.com/wp-content/uploads/2012/02/The-hard-leaves-type-of-sansevieria-600x668.jpg.

For the wall behind, I would suggest multiple pictures with offset framing. The pictures can be of different colors that you think match well or that are pleasing. You can either do multiple smaller pictures, or a few larger ones, depending on your personal preferences. I would tend towards the 2-3 larger colorful pictures in reddish tones as you have many individual beige/brown furniture pieces. IKEA has a nice picture that I think would look great here as a starter, but it's not so cheap (http://www.ikea.com/us/en/catalog/products/60208907/).
Or a cheaper, but still good, starter (http://www.ikea.com/us/en/catalog/products/10149717/). Flowers would look great here to soften up the straighter lines of your furniture.

I know pinterest can be kind of cheesy, but here are some good ideas on what i mean: http://pinterest.com/jessicab1/inspiration-art-walls/

Alex first, thank you both.

we thought about the rugs and we still searching them. i think about light/pale, eventualy white with little black rug color/pattern. i consider the tall plant at the end of the credenza a very good ideea. i also think about an accent wall in the dining area like reddish brick or in a more elegant way, brown stone (i personaly prefer the brick which makes a pleasant color contrast with a dark blue catwalk chair covers, i don't know...).
About the wall art, i think 3 big 1x1m canvases is the best ideea but i have difficulties in finding them.

olldbobbi This would be amazing over your credenza: [houzz=
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sapnapg It is great space except that the empty walls make it look a little cold. Suggest a painting or a mirror with hand-work around it to shift focus to the living area, rugs to demarcate space and if you insist on partitioning it, how about bamboo curtains(not single sheet - but multiple longitudinal strips forming a curtain so that you can move them) or folding doors made of wood that would let air . Also, you can move them if you want more space for a party.

Alex @olldbobbi: perhaps something minimalist?

Alex @sapnapg: i agree with you. looks cold. that's why i posted dilemma.
on the partitioning problem, it's difficult to use courtains because the dining table & one chair exceeds the "separation line" and beyond that the table is extensible. that's why i'm thinking on one tall plant. it gives me a simple delimitation and more space.

Otrada LLC Ok. Great Space. First thoughts are to move the credenza a bit to the Dining area or the Living area. It may just be the camera angle of the photo, but right now I'm not sure it's helping to define either space. I agree with rugs to soften the hard finishes and create two spaces.

I also notice that every item is a rectangle: Sofa, dining table, credenza etc. so you may want to add some organic shapes for light fixtures, mirrors, and accessories. What is your style beyond modern? If it was my space, I'd move the credenza to the dining area (if it fits?), place a large round mirror above it for some drama and glam. I'd then select a huge original art piece for the Living Area to be hung almost floor to ceiling. It would be something to expand the width of the room with lots of color. I'd let the art be the focal point for the room. Something that feels alive. Abstract. Whatever speaks to you since you will be the best judge of the type of art that you like. If you can't make the budget work for original art, perhaps a fabric wrapped canvas, tribal blanket, graffiti art on canvas - just go large and make a statement.

Alex @Otrada: the credenza cannot be moved in the dining area because the crossing space there is limited by the kitchen resulting in a narrowed crossing space in the kitchen & terrace area. and even if i'm trying to make this move, there's a radiator under the split (AC) and the credanza will stand very very close to this radiator.

olldbobbi Alex, if you have no plans to move the credenza, this wallpaper would be stunning framed in behind it: [houzz=
]

Otrada LLC Alex, I would then move it down a bit to center the Living Area. Large Artwork, funky lamps, and a bit of color are needed around it so it won't feel like it's floating on that wall. If you like a mod look, I'd add a rug in a solid color, but with texture just to the Living area. It needs something bold and warm. I'd finish it off with a 70's inspired arched floor lamp and some new accent pillows.

The Dining area can go without a rug because most times it just looks cleaner in an open concept layout, plus they tend to get in the way of the chairs' movement. On the wall in the dining area you could add: Mirrors in a vertical pattern (glam),a narrow planter with tall bamboo (organic), or perhaps a group of 9 photography prints (global). It just depends on your style. Great terrace space as well.

I probably would not wallpaper the entire wall.

Otrada LLC Dining area will need an amazing pendant light fixture too!

Alex i agree with pendant light in the dining area and something else in the living area. The credenza is perfectly centered in the living area. it's just an optical problem. what do you think about an accent wall in the dining area. i was thinking about brick (natural, reddish) texture. and 3 big 1x1m (40x40") canvases in the living area just above the credenza.

Otrada LLC Dining-Adding faux brick or an image? Do you have a picture?
Living- what is the ceiling height? Yes, as long as it will bring your eye up towards the ceiling. Vertical height needs to be accentuated to diminish how long that wall appears.

Alex Dining - Faux brick. I haven't searched a model yet. Just an ideea.
Living - Ceiling height is 2,70m (106").

COASTROAD Hearth & Patio Need a room divider? I am biased, but I like something like this:

Prime Walls Your space is lovely! I enjoy the idea of the rugs to divide the space, and also sperate wall treatment.
Consider maybe a linear mural above the credenza, and a feature wallpaper on the dining room side.
You mentioned brick and therefore having a more rustic appearance, here are a few suggestions for an easy and affordable way to achieve that look.

http://www.primewalls.com/products_residential/index.php?pattern_id=&primarycolour=&accentcolour=&style=&book_id=35

Primewalls.com

Chandani If you are still having trouble finding large scale art, there are some great sites to peruse. Here is one:
http://www.art.com/me/ihokb8ls6ms8hbulp16cxw2/

Ikea also has large prints, though, I admit, you probably don't want art that you are likely to see elsewhere! If you are creative, you can purchase large stretched canvases at an art supply or craft store and paint your own!

Alex @Chandani: you're right. Maybe the best option is to make sometihing custom in a minimalistic way.

Otrada LLC Alex, I would spend on the Living room first with the rug, pillows, lamp, and artwork. After this is complete, I would review the dining area to see if it needs to be further defined by a wall treatment. Commercial textured or patterned Wallcovering may always be added. Although I'm not sure the space really needs it. It feels light and airy so I would work with that first.

A cubicle curtain track tucked behind the soffit with a linen drape would also offer privacy and soften the dining area.