Preface: Initially spawning in
As a preface to this topic, I want to explain what will happen when you spawn in your custom interior (ID 200) for the first time. In most cases, you need to wait for a server restart after having clicked pay/accept button on UCP for an interior change to take effect. Upon entering, you will spawn under the map, then most likely fall to your death on a platform (this is the camper interior which is constantly spawned in under the map). Use /fixrevive and jump off, you will eventually land in middle-city, still within your own dimension of the GTA map. From here, you can decide where you would like to map. Most people map just above their property because of the bugginess that comes with mapping inside the actual property. You can either ask an admin for assistance getting to the roof, or build your way up there. From here, you can put down a floor and use /pentrance to set the entry spawn point.
For a stand-alone property, see #2.
1. The size of the default Rockstar interior that the apartment comes with is considered. For example, the Fancy apartment interior is more spacious than the blue (2 bedroom) apartment interior. The blue apartment is more spacious than the green apartment (1 bedroom), and the green apartment is more spacious than the motel room.
Some approximate character step counts, helpful when the exterior features are too generic or there are too many apartments per floor to judge:
Fancy/Luxurious Red and Black apartment (Interior ID 2): 20 by 20-30 steps, depending on exterior size
Basic Blue Apartment (2 bedrooms, Interior ID 6): 15 by 10-15 steps, depending on exterior size
All the 1-Bedroom Interiors (ID 7 - Green Apartment, ID 11 - Stairs Apartment, ID 79 - Janitor's Apartment) : Maximum 10 by 15 steps
Motel Room (ID 33): Very small, maximum 10 by 10 steps
2. The next thing to consider is the overall size of the property compared with the number of apartments per floor. There are a few ways to judge the size of a property. You can count the number of character steps while walking along the perimeter and do some math to determine how much space would belong to your apartment. I personally prefer this method when there is a small number of apartments per floor because it's the most accurate. You can also use the item "Massive White and Red striped Tile" (/sf massive) for large properties; rather than counting the steps, you can count the number of squares or stripes. If it's a stand-alone property, you can also just build above the property which makes it easy to judge the size while mapping out your floors.
3. Another way to judge how much space would belong to your apartment is taking a good look at the exterior features of the property. Many apartment buildings will have several identical sets of windows and balconies per floor, helping you to judge approximately where each apartment would begin and end. While in your interior's dimension, you can spawn in things like pillars, columns or walls and place them where you think the borders of each apartment would be - here's an example:
4. The option also exists to choose one of the pre-built custom interior skeletons available on the UCP Interior Change page, bottom of the list. Several skeletons with walls and some basic structures are built which take the ? out of sizing. You can choose based on location or style. You need either donator status or permanent mapping slots to complete these, and all the furniture can be edited, unlike Rockstar interiors.
The last thing to note is that while custom mapping, you are expected to adhere to the exterior of the building in not only size but style (is it run-down? consider the location you're in), and windows.
If your apartment doesn't have glass walls on the outside, you shouldn't be mapping floor-to-ceiling windows.
If your apartment has many apartments per floor, you should only map windows on one to two sides. Realistically, you would have neighboring apartments on either side and a corridor on the entry-side wall.
If your apartment is on the second floor of a high-rise building, you shouldn't be mapping on the roof just to have a high-up view.
Be careful with water effects and lavish decoration and keep in mind that if your property is in a run-down neighborhood, it should reflect that.
Basements need to be approved by a PM admin before they are mapped.
Indoor pools or jacuzzis are often not allowed in apartment buildings, but may be considered for a business. Always check with a PM admin if you're unsure.