Making an epic spell is about presenting a convincing mechanical argument which the DM deems to fall within a reasonable margin of power for the character and his or her campaign; there is no single right way. Certain precedents are contained within official epic spells themselves.

FWIW, here's how I'd do it:

Look at Pestilence. It contains a precedent: a nonepic spell (contagion) which has no counterpart as an epic seed effect is appended as a "virtual seed" to the [Afflict] seed. [Afflict] might be viewed as a "carrier seed" for *[Contagion]. Its DC modifier (+21) is consistent with that of other seeds. Various factors are then applied to it.

You therefore need the [Conjure] seed, to which a magnificent mansion effect is appended; the basic parameters of magnificent mansion (as cast at 20th-level) become those upon which the final epic spell factors work. The *[Magnificent Mansion] seed should have a Spellcraft DC modifier of +27, as magnificent mansion itself is a 7th-level spell.

So far:

[Conjure] (+21): Acts as carrier; as description
*[Magnificent Mansion] (+27) Sets basic spell parameters as magnificent mansion at 20th-level (60 x 10-ft cubes; 75ft range; 40hrs duration)

= +48

Now employ factors.

Let's say you want to increase the area of a regular MMM by 2,000%. Normally, with the magnificent mansion you'd get 60 x 10-ft cubes. Now, you get 1260 x 10-ft cubes. This incurs a +80 Spellcraft DC modifier.

So far, the Spellcraft DC is +128. If you want a permanent version, you're looking at the x5 multiplier at this point. This is where it can get ugly.

If, instead you want to extend the spell's duration to 1 week (+300%), that's only another +6. If you're not in a hurry to cast it, you can extend casting time by 10 mins (-20 mitigation). You will have to burn XP or use a fancy component to get the DC lower than 114.

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