I have several different solutions that I'd suggest to this problem.
Genesis is a level 9 Shaper power that creates a new demiplane on the Astral Plane. Manifest the power, wait a few months, and you have a plane that's a few hundred feet wide, permanent, and shaped however you want it. You need a way of travelling between planes to get there, but other that, you have a permanent plane that you can access whenever you want. This isn't quite an extradimensional space, but it has the advantage of being strictly RAW.
Permanency with Magnificent Mansion
The only 2 spells in 3.5 that I know of that allow you to make extradimensional spaces are Rope Trick and Mage's Magnificent Mansion. Rope Trick is clearly insufficient, but Mage's Magnificent Mansion would be at least large enough for what you intend. Mage's Magnificent Mansion isn't on the normal list for Permanency, but there's no real reason that it couldn't be. Since it's a level 7 spell, it would probably have a minimum level of 15 and cost 2500 XP (17,500 gp in Pathfinder) to make permanent.
Permanency with a homebrew spell
The way I would accomplish this is with a homebrew extradimensional space spell. Rope Trick, at level 2, is the smallest extradimensional space, which has no actual internal dimensions; it has just enough space to hold the creatures inside it, and no more. Mage's Magnificent Mansion is at the opposite end of the scale: it's huge, and has nice side effects, but is much higher in level. For this homebrew spell, I'd put it somewhere in the middle. I like level 5 for this effect. 5 is where you get Secret Chest, Wall of Stone, and Mage's Sanctum, which are some of the standard "make a secure shelter" spells that are more secure than a locked room. Here's the spell I'd make:
Mage's Extradimensional Hut
Level: Sor/Wiz 5 Components: V, S Casting Time: 1 Standard Action Range: Close Effect: Extradimensional mansion, up to 1 10-ft. cube/level (S) Duration: 2 hours/level (D) Saving Throw: None Spell Resistance: No
You conjure up an extradimensional dwelling that has a single entrance on the plane from which the spell was cast. The entry point looks like a faint shimmering in the air that is 4 feet wide and 8 feet high. Only those you designate may enter the mansion. You may open it again from your own side at will. The atmosphere is clean, fresh, and warm.
You can create any floor plan you desire to the limit of the spell’s effect. The place is unfurnished, and any walls are made of bare wood or stone, your choice.
Since the place can be entered only through its special portal, outside conditions do not affect the mansion, nor do conditions inside it pass to the plane beyond.
This spell can be made permanent with the Permanency spell. It has a minimum caster level of 9, and costs 500 XP (2,500 gp in Pathfinder).