Create a homebrew spell, filling out enough information to be able to save it and specifically with "Duration Type" set to "Time", "Duration" set to 10 minutes, and "At Higher Levels Scaling" set to true.
Click the "Add a Higher Level" button.
Set "Scaling Level Value" to 1.
Set "Scale Effect" to "Extended Duration".
Set "Fixed Value" to 10.
Create a character of a high enough level to prepare the spell and prepare it for them.
Look at the entry for the spell in the sections for the spell slots of a higher level. Instead of showing some indication of the duration being extended, a blue "Special:" with no information is rendered instead.
Old thread but this is how I made it work, my spell's level is 6th and it has base duration 1 minute. For extending the duration for each 7th, 8th, 9th, I made a separate entry. Make sure to select "Spell Level" in the "Higher Level Scaling Type" so that the "Level" in the "At Higher Levels" as seen in the linked screenshot is for the spell slot used.
P.S. Extended Area DOES work with "Spell Scale" and fixed values for incrementing for each higher level automatically. Maybe that's because a spells area/range is always in the same unit (feet), maybe there are gremlins, can't say for sure.
P.P.S. Turns out that if you need only Ex. Area then you can pick "Spell Scale" and use fixed value for increments. BUT if you need both Ex. Area and Ex. Duration you need to select "Spell Level". And because that setting is not tied to each entry in the "At Higher Levels" but the "At Higher Levels" category in its entirety you need to individually input each Ex. Area value. Homebrewing is a wonderful hobby.
That makes it appear nicely in the homebrew section but still does not have the duration change pass onto the character sheet when the spell is placed in the Wizard's spellbook.
That is because it is text only. Those kinds of interactions have not been programmed yet. They are redesigning the system to accommodate things such as you suggest. The best place to give that feedback is here:
Steps to reproduce:
Also seeing this bug. example spell: https://www.dndbeyond.com/spells/452502-desert-thermal
Old thread but this is how I made it work, my spell's level is 6th and it has base duration 1 minute. For extending the duration for each 7th, 8th, 9th, I made a separate entry. Make sure to select "Spell Level" in the "Higher Level Scaling Type" so that the "Level" in the "At Higher Levels" as seen in the linked screenshot is for the spell slot used.
P.S. Extended Area DOES work with "Spell Scale" and fixed values for incrementing for each higher level automatically. Maybe that's because a spells area/range is always in the same unit (feet), maybe there are gremlins, can't say for sure.
P.P.S. Turns out that if you need only Ex. Area then you can pick "Spell Scale" and use fixed value for increments. BUT if you need both Ex. Area and Ex. Duration you need to select "Spell Level". And because that setting is not tied to each entry in the "At Higher Levels" but the "At Higher Levels" category in its entirety you need to individually input each Ex. Area value. Homebrewing is a wonderful hobby.
–Hangikjet
That makes it appear nicely in the homebrew section but still does not have the duration change pass onto the character sheet when the spell is placed in the Wizard's spellbook.
That is because it is text only. Those kinds of interactions have not been programmed yet. They are redesigning the system to accommodate things such as you suggest. The best place to give that feedback is here:
https://www.dndbeyond.com/forums/d-d-beyond-general/d-d-beyond-feedback/30499-request-and-vote-for-new-features-for-ddb
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