You've seen all of those really nifty-looking mansions on The Sims 2 lot exchange, right? Of course you have! Though they look extraordinarily complex to build... they probably are. But, you don't need to hack your game in order to build a really nifty house of your own in The Sims 2.
Of course, the principles behind building stuff in The Sims 2 and The Sims 3 are fundamentally the same -- so, if you'd prefer to use The Sims 3, that's fine, too. I prefer Sims 2, though, mostly because my computer can actually run it with a certain degree of reliability. That and I think Sims 2 had niftier stuff than its successor has.
Right. On with the point.
Most Sims players just improvise when they build houses (I certainly do) under the intention of populating it with a multi-generational family as quickly as possible. However, players who play specifically to build have a different mindset -- they prefer to fill their neighbourhoods with houses... perhaps to leave them standing empty, never to be used. These are the houses that get uploaded to the exchange, get a hundred thousand downloads, and possibly get used by Sims Division in the next game.
Just like in real life, showcase houses get an intense amount of planning. How large is our lot? How much space do we want for landscaping? How high up are we willing to go? Would this room be more efficient if it were one tile wider? These are all questions that a showcase builder needs to ask, even before startup!
In cases such as this, I find graphing paper to be indispensable. You can go to your local office-supply store and find notebooks or individual reams of graphing paper of varying sizes and grid densities -- for large lots (the >5x4 kind), you'll no doubt need the highest grid density in the store.
Next, you'll need inspiration. My advice is to simply drive about in the wealthier areas of your town. Whilst there, take note of the sorts of building materials you see (bricks, stucco, siding) and how much space there is between the pedestrian sidewalk and the house. Don't look at construction so much yet... most of the newer high-end houses are built with things that The Sims series does not have, like walkout basements and skylights.
Now, drive about in the somewhat lower-class areas of town. Not like the ghetto or the projects, but something more working-class... something in between SUV-land and paved paradise. Why this area? Because you're more likely to find houses that you can copy. Take pictures of houses that you think you'll be able to build, then later, using existing game objects as a point of reference, put an approximation of what you think the house might look like onto your graphing paper. You don't need to know the structure's exact dimensions... just guess. Fill in the floorplans to suit your own taste. Whatever you end up with will be fine.
If you're more into James Bond-style photo reconnaissance, you can go to open houses or make appointments with realtors to see the inside of houses for sale. You'll waste their time, but it will be well-worth your time (I'm not actually encouraging you to do this... it's just something to think about).
Rather than copy these floorplans directly onto Build Mode grid-tiles, find some that are similar to each other (maybe two or three) and combine them! Maybe wrap a hacienda around a tudor and add an outbuilding of three stacked ranch-houses.
Of course, sometimes, you'll come across a structure which needs no alteration at all. It's just so nifty that you can't improve upon it. In that case, just copy it down there!
Another thing you can do is to combine pre-existing game floorplans. If you take that narky spanner-like roof off the Modern Masterpiece, you may be able to wedge the lot of it into the Monty Family's house.
But then, these suggestions have been largely about copying. Once you have a promising design, adapt it! There's no boxy thing that can't become new-age with a few well-placed diagonal walls.
Improvise on it! Move some of the rooms about, maybe. Perhaps you like the kitchen from one house and the sitting-room from another. Really, it's amazing just how much like LEGOs Sim-houses actually are.
But, with all of this combining and copying and improvisation, keep in mind one very important thing. One which all sensible buyers look for in a house: curb-appeal. How nifty the structure looks from the street. Curb-appeal will make or break your house's popularity. It could be all high ceilings and rooms fit for kings on the inside, but if it's just a box on the outside...
Fortunately, curb-appeal is a bit more forgiving in The Sims 2 than it is in real life, mainly because of the angle from which the house's picture is taken: high up, pointing down. If your building has a great deal more in the back than it does up front, that can be partially seen. For instance: you built a box in the front, but you added onto the back. It may look boring from street-level, but a bird's-eye view shows your structure for what it really is.
Landscaping will also help curb-appeal. Terraced flower-beds look great, particularly when paired with stair-stringers. If your house is at the top of a hill, use the stair-stringers to automatically create terraces, which you can then extend with the terrain-levelling tool. Also, depending on which side of the lot you built your house on, symmetry can be a very good thing or a very bad thing. If the house is almost exactly in the centre of the lot, you'll want to put serious consideration in to landscaping symmetrically (that is, the same on both sides). However, if the building ended up mostly on the side of the lot, you don't want a lot of symmetry. A water feature on the empty part of the lot would make an excellent counterpoint to the house, to which you could point using flower-beds or shrubs.
Trees, though, are sort of the black-sheep, the white elephant, the pink hippopotamus of the landscaping set. No matter how centred or how far aschew your house may be, no one likes symmetrically-planted trees. Try to resist placing trees in the same place on either side unless absolutely necessary (for example -- a front garden arboretum would require symmetry).
On the subject of landscaping, you can make your house look hacked without actually using any hacks. This is done through creative usage of the "moveObjects" code. Place a palm tree in the middle of a fountain or a rubber-tree plant inside a shrub. Put a gnome on a flower bed (though this may make the flowers unusuable).
The landscape isn't the only part of the lot that can benefit from codes like this. In FreeTime onwards (not that there's much after that), you can use the "moveObjects" code and the bracket keys ([ and ]) to place wall-hanging objects over each other. Also, it's sort of a long and clunky code, but "boolProp allow45degreeangleofrotation true" will permit you to rotate all objects in 45-degree increments, putting things on the diagonal. However, whilst it may look good, Sims won't be able to interact with certain diagonal objects.
Finally, if you do decide to upload your creation to the Exchange or to make it available on your website -- just as a courtesy, you should furnish the kitchen and any bathrooms in the house. If you're the type with control issues, you may want to consider putting a few function-suggestive objects in rooms that you're dead-set on serving a particular purpose (put a bed and an armoire in the bedroom, or a desk and some bookshelves in the study, et cetera). However, you'll find that most people prefer to furnish their downloaded houses per their own tastes. You may think that this should be the dining-room... well, someone else may think that it would be better-served as a home-office.
Also, people may prefer that you not include Sims with your house. Unless you're doing a story, only share empty houses. If you upload a pre-populated house, any furnishing that you may have done will be lost when the next guy moves your Sim out.
Off the subject of houses for a moment...
Showcase lots apply to community buildings as well (but only in The Sims 2). So you don't want to make another clothing shop or restaurant. That's fine -- you may be surprised at the demand for lots which serve no practical purpose! These "filler" lots consist of office buildings, warehouses, construction sites, schools, or any other structure that would lend an air of realism to a city. Obviously, these buildings can't ever be used for their intended purposes, but people will download them, nonetheless. Maybe they're like me and wish that SimCity and The Sims would merge one of these days -- filler lots do create a very impressive illusion.
Anyway, give those ideas a try! You might end up being the next Frank Lloyd Wright! Or, at least, Will Wright...